test: use subprocesses for each test file (#29016)

* ci(tests): install ripgrep from prebuilt tarball instead of apt

apt-get update + install of ripgrep takes ~4 min on the GHA Ubuntu
runners (the apt-get update against archive.ubuntu.com is the slow
part; ripgrep itself is small). Switching to the upstream musl
binary tarball cuts the step to a few seconds.

- Pinned to ripgrep 15.1.0 with sha256 verification (same hash as
  published in the releases sha256 sidecar file).
- Drops the `rg` binary into /usr/local/bin so it is on PATH for
  every subsequent step without GITHUB_PATH manipulation.
- Applied to both the test and e2e jobs in tests.yml.

* fix(cli): compile syntax check to tempdir, not source __pycache__

`_validate_critical_files_syntax` runs `py_compile.compile()` on each
critical bootstrap file after a successful `git pull`. The default
`py_compile` writes the resulting `.pyc` next to the source under
`__pycache__/`, which causes two real problems:

1. Parallel test workers walking the same source tree (e.g. running
   the suite under per-file process isolation) can race against each
   other on the `__pycache__` write — manifests as flaky 'directory
   not empty' errors during teardown.
2. In production, the post-pull syntax check leaves a `.pyc` behind
   that the next interpreter run might pick up — fine when the
   interpreter version matches, sketchy if it doesn't.

Fix: write the compiled output to a `tempfile.TemporaryDirectory()`
that's discarded on function exit. We only care about the compile-or-not
signal, not the artifact.

* test(runner): per-file process isolation, drop manual state reset + xdist

Replace fragile manual _reset_module_state test fixtures with robust
per-file subprocess isolation. Each test file runs in a fresh
`python -m pytest <file>` subprocess via ThreadPoolExecutor. No xdist,
no custom pytest plugin, no shared worker state.

Key changes:
  * scripts/run_tests_parallel.py — new runner: discovers test files,
    runs N in parallel via ThreadPoolExecutor, captures stdout per file,
    treats exit code 5 (no tests collected) as pass, kills all children
    on exit. Change from cpu_count to cpu_count*2. The runner is
    I/O-bound (waiting on subprocess.communicate() from pytest children)
    The parent process does almost no CPU work, so 2x oversubscription
    keeps more pipes full. When a file fails, immediately show the last
    30 lines of pytest output (stack traces + FAILED summary) plus a
    ready-to-copy repro command:
      python -m pytest tests/agent/test_auxiliary_client.py
  * scripts/run_tests.sh — delegates to run_tests_parallel.py
  * .github/workflows/tests.yml — test step: python
scripts/run_tests_parallel.py
  * pyproject.toml — drop pytest-xdist, pytest-split; simplify addopts
  * tests/conftest.py — remove ~200 lines of manual state-reset fixtures
  * AGENTS.md — update Testing section for per-file design

* test(runner): speed gateway test antipattern scan up

* fix(test): web search provider plugin test missing xai

* fix(tests): make 14 test files pass under per-file subprocess isolation

Tests that relied on cross-file state pollution from xdist workers
fail when run in isolation (per-file subprocess model). Root causes
and fixes:

Tool registry not populated:
  - test_video_generation_tool_surface_matrix: add discover_builtin_tools()
  - test_web_providers_brave_free/ddgs/searxng/general: autouse fixtures
    registering all 8 bundled web providers, reset after each test
  - test_website_policy: same provider registration pattern
  - test_web_tools_tavily: same pattern across 3 dispatch test classes
  - Also add is_safe_url/check_website_access mocks where SSRF check
    blocks example.com (DNS resolution fails in isolated envs)

Stale check_fn cache:
  - test_kanban_tools: invalidate_check_fn_cache() + _clear_tool_defs_cache()
    in both kanban guidance tests (prior test cached False for kanban_show)
  - test_discord_tool: cache invalidation in setup/teardown
  - test_homeassistant_tool: invalidate_check_fn_cache() before registry queries

Module-level state pollution:
  - test_auxiliary_client: autouse fixture clearing _aux_unhealthy_until cache
  - test_skill_commands: set_session_vars() instead of patch.dict(os.environ)
    (ContextVar takes precedence over os.environ)
  - test_dm_topics: overwrite sys.modules + separate telegram.constants mock
    + force-reimport of gateway.platforms.telegram
  - test_terminal_tool_requirements: removed duplicate class declaration,
    autouse _clear_caches fixture

* change(tests): run_tests.sh explicitly includes env vars

instead of manually dropping some vars, now we just only include some

* fix(tests): 5 more isolation/NixOS fixes

- test_approval_plugin_hooks: isolate HERMES_HOME so real user's
  command_allowlist doesn't short-circuit the approval path
- test_google_chat: skipif when Platform.GOOGLE_CHAT not in enum
  (feature not merged on this branch)
- test_write_deny: test systemd prefix against tmp_path instead of
  /etc/systemd which resolves to /nix/store on NixOS
- test_pty_bridge: use shutil.which('cat') instead of /bin/cat
  (doesn't exist on NixOS)
- profiles.py: rmtree onexc handler chmod's parent dirs too, fixing
  profile deletion when copytree preserved read-only modes from
  nix store

* fix(tests): clear unhealthy cache in autouse fixture for auxiliary_client

* fix(tests): skip send_message when telegram not installed; handle missing worker_id in browser_supervisor

* fix: py3.11 rmtree onexc compat + belt-and-suspenders unhealthy cache clear for expired codex test

* fix: address PR #29016 review feedback

- Remove tracked .pytest-cache/ artifact and add to .gitignore
- Fix stale 'xdist worker' comment in conftest.py
- Deduplicate web provider registration into tests/tools/conftest.py
  shared helper (register_all_web_providers), replacing 8 copy-pasted
  blocks across 6 test files
- Update PR description: remove stale recovered-test-files claim,
  fix worker count to match code (cpu_count*2)

* fix: eliminate race in stale-cache achievements test

The background scan thread could complete and overwrite _SNAPSHOT_CACHE
before evaluate_all() returned the stale data — only 10 fake sessions
made the scan finish instantly. Added scan_delay param to _FakeSessionDB
and set it to 2s in the stale-cache test so the background thread can't
win the race.

AGENTS.md

@@ -1013,17 +1013,39 @@ def profile_env(tmp_path, monkeypatch):
 
 **ALWAYS use `scripts/run_tests.sh`** — do not call `pytest` directly. The script enforces
 hermetic environment parity with CI (unset credential vars, TZ=UTC, LANG=C.UTF-8,
-4 xdist workers matching GHA ubuntu-latest). Direct `pytest` on a 16+ core
-developer machine with API keys set diverges from CI in ways that have caused
-multiple "works locally, fails in CI" incidents (and the reverse).
+`-n auto` xdist workers, in-tree subprocess-isolation plugin). Direct `pytest`
+on a 16+ core developer machine with API keys set diverges from CI in ways
+that have caused multiple "works locally, fails in CI" incidents (and the reverse).
 
 ```bash
 scripts/run_tests.sh                                  # full suite, CI-parity
 scripts/run_tests.sh tests/gateway/                   # one directory
 scripts/run_tests.sh tests/agent/test_foo.py::test_x  # one test
 scripts/run_tests.sh -v --tb=long                     # pass-through pytest flags
+scripts/run_tests.sh --no-isolate tests/foo/          # disable subprocess isolation (faster, for debugging)
 ```
 
+### Subprocess-per-test isolation
+
+Every test runs in a freshly-spawned Python subprocess via the in-tree plugin
+at `tests/_isolate_plugin.py`. This means module-level dicts/sets and
+ContextVars from one test cannot leak into the next — the historic
+`_reset_module_state` autouse fixture is gone.
+
+Implementation notes:
+
+- The plugin uses `multiprocessing.get_context("spawn")`, which works on
+  Linux, macOS, and Windows alike (POSIX `fork` is not used).
+- Per-test overhead is ~0.5–1.0s (Python startup + pytest collection). xdist
+  parallelism amortizes this across cores; on a 20-core box the full suite
+  finishes in roughly the same wall time as before, but flake-free.
+- `isolate_timeout` (configured in `pyproject.toml`) caps each test at 30s.
+  Hangs are killed and surfaced as a failure report.
+- Pass `--no-isolate` to disable isolation — useful when debugging a single
+  test interactively, or when you specifically want to verify state leakage.
+- The plugin disables itself in child processes (sentinel envvar
+  `HERMES_ISOLATE_CHILD=1`), so there's no fork-bomb risk.
+
 ### Why the wrapper (and why the old "just call pytest" doesn't work)
 
 Five real sources of local-vs-CI drift the script closes:
@@ -1034,7 +1056,7 @@ Five real sources of local-vs-CI drift the script closes:
 | HOME / `~/.hermes/` | Your real config+auth.json | Temp dir per test |
 | Timezone | Local TZ (PDT etc.) | UTC |
 | Locale | Whatever is set | C.UTF-8 |
-| xdist workers | `-n auto` = all cores (20+ on a workstation) | `-n 4` matching CI |
+| xdist workers | `-n auto` = all cores | `-n auto` (safe — subprocess isolation prevents cross-worker flakes) |
 
 `tests/conftest.py` also enforces points 1-4 as an autouse fixture so ANY pytest
 invocation (including IDE integrations) gets hermetic behavior — but the wrapper
@@ -1042,15 +1064,21 @@ is belt-and-suspenders.
 
 ### Running without the wrapper (only if you must)
 
-If you can't use the wrapper (e.g. on Windows or inside an IDE that shells
-pytest directly), at minimum activate the venv and pass `-n 4`:
+If you can't use the wrapper (e.g. inside an IDE that shells pytest directly),
+at minimum activate the venv. The isolation plugin loads automatically from
+`addopts` in `pyproject.toml`, so you get the same per-test process isolation
+either way.
 
 ```bash
 source .venv/bin/activate   # or: source venv/bin/activate
-python -m pytest tests/ -q -n 4
+python -m pytest tests/ -q
 ```
 
-Worker count above 4 will surface test-ordering flakes that CI never sees.
+If you need to bypass isolation for fast feedback while debugging:
+
+```bash
+python -m pytest tests/agent/test_foo.py -q --no-isolate
+```
 
 Always run the full suite before pushing changes.