Two unrelated but co-located fixes to scripts/run_tests.sh:
1. pytest-split bootstrap (#22401): the script tried '$PYTHON -m pip
install pytest-split' on first run, but uv-created venvs ship without
pip. Result: 'No module named pip' before any test ran. Add a uv
fallback (uv pip install --python $PYTHON), keep pip as a secondary
path, and emit a clear error pointing at 'uv pip install -e ".[dev]"'
when neither is available. Also declare pytest-split in
pyproject.toml dev extra so a normal '.[dev]' install provisions it.
2. HERMES_CRON_SESSION leak (#22400): the hermetic env scrub already
unsets HERMES_GATEWAY_SESSION and HERMES_INTERACTIVE but missed the
sibling HERMES_CRON_SESSION. When run_tests.sh is invoked from a
Hermes cron job, that variable leaks into pytest, flipping
tools/approval.py into cron-deny mode and breaking
tests/acp/test_approval_isolation.py and friends.
Closes#22400.
Closes#22401.
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