docs: improve test suite documentation

[man4ish]

Summary

• Adds module docstrings, Developer: Manish Kumar <manish@omnibioai.org> attribution, and assertion-specific docstrings across the security-audit test suite (46 files under tests/).
• Coverage: modules 46/46, attribution 46/46, Test* classes 12/12 (1 existing preserved, 11 added), test_* functions/methods 478/478 (109 existing docstrings preserved, 369 added). Fixtures and reusable helpers are documented where they carry non-obvious setup.
• Descriptions are scoped to the exact tested invariant (e.g. "Deny a dangerous command with DangerousCommandError specifically") — no broad security/compliance claims added anywhere.

Documentation only

• Executable AST differences vs main: 0 across all 46 files (docstring nodes normalized out before comparing).
• No source, configuration, or dependency files changed. Only tests/*.py is touched (+711/−10; the 10 removals are existing module-docstring closers moved down to append attribution).
• ruff check on the changed files: identical finding count (15) before and after, confirmed via git stash/stash pop — nothing introduced by this change.
• Boilerplate audit caught 6 duplicate docstrings (legitimate parallel behavior across files, e.g. identical 401/403 wording on two different routes); each was reworded to name its own endpoint or function. 0 duplicates remain.

Safety

• B0_TEST_MYSQL_ROOT_URL / B0_TEST_REDIS_URL were left unset throughout, so every guarded real-backend test (including the retention/immutability suite tied to a documented 2026-09-16 production incident) skipped cleanly rather than touching anything real.
• test_redis_acl_safety.py's Docker-based tests are self-contained: a freshly named, random-port, --rm throwaway redis:7-alpine container that explicitly refuses the production port if Docker allocates it by coincidence. Verified no leftover containers after the run.

Test plan

• Syntax check on all 46 files: 0 failures
• Focused (Redis ACL safety + MySQL guard) tests before and after: 94 passed, 4 skipped (identical)
• Full suite before and after: 495 passed, 39 skipped (identical skip set)
• git diff --check clean

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