Address Copilot review on PR #17569:
1. _resolve_safe_cwd never tested the filesystem root because the loop
exited when `os.path.dirname(parent) == parent`, which is true once
`parent == '/'`. Restructure so the root is checked before the
self-equal exit. Adds `test_returns_root_when_only_root_exists` —
regression-guarded by reverting the loop and watching it fail.
2. The fake `Popen.stdout` was a `MagicMock`; `BaseEnvironment._wait_for_process`
calls `proc.stdout.fileno()` then `select.select`/`os.read` against it,
which raised `TypeError: fileno() returned a non-integer` (visible as a
thread exception in test output) and could in theory read from an
unrelated real fd. Hand `fake_popen` a real `os.pipe()` with the write
end pre-closed so the drain loop sees EOF immediately. Helper records
each fd so the test cleans up after itself.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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