fix(tests): defer live module env load to test execution, not collection

[wali-reheman]

fixes #80343

What

Move _load_user_env() out of module scope and behind an autouse fixture that
runs after _hermetic_environment. The old module-level call ran at pytest
collection time, before the credential scrubber, leaking developer-local
env vars (e.g. SEARXNG_URL) into os.environ and causing non-deterministic
test failures.

Why

When test_sequential_chats_live.py was collected, _load_user_env() read
~/.hermes/.env and wrote into os.environ immediately — before any pytest
autouse fixtures had a chance to scrub credential-shaped variables. With
SEARXNG_URL leaked, _get_backend() in test_web_tools_config.py returned
searxng instead of firecrawl, causing those two tests to fail on machines
that had SEARXNG_URL configured locally.

How

The fix defers the env load to fixture time, not collection time. An autouse
fixture _live_test_env calls _load_user_env() after _hermetic_environment
has already run — so live tests still get OPENROUTER_API_KEY for execution,
but collection is clean.

Also adds tests/test_env_leak_guard.py: a regression test that walks every
test module's AST and fails if any module writes a file-read result into
os.environ at module scope.

Verification

# The original reproducer (before fix — 2 failures, 1 skip):
pytest tests/run_agent/test_sequential_chats_live.py \
  tests/tools/test_web_tools_config.py::TestBackendSelection::test_fallback_no_keys_defaults_to_firecrawl \
  tests/tools/test_web_tools_config.py::TestCheckWebApiKey::test_null_backend_value_does_not_crash -q
# 2 passed, 1 skipped

# Regression test catches the pattern:
pytest tests/test_env_leak_guard.py -q
# passes

[alt-glitch]

This was generated by AI during triage.

Duplicate of #80343: both repair the same module-scope developer .env load during pytest collection. #80343 is the earlier, narrower implementation; please consolidate any additional regression coverage there.