docs: improve test suite documentation

[man4ish]

Summary

• Adds a module docstring and Developer: Manish Kumar <manish@omnibioai.org> attribution to all 21 test modules under tests/ (including the previously empty tests/__init__.py and tests/conftest.py).
• Adds a docstring to all 160 test_* functions (130 were undocumented), each stating the specific behavior the test asserts. Documents all 12 fixtures and 12 helpers. Existing docstrings are preserved verbatim.
• Documentation only: no imports, assertions, fixtures, mocks or application code changed. 290 lines added, 0 removed.

Verification

• AST equivalence: for every modified file, HEAD vs. working tree with docstring-only expression nodes stripped gives 0 executable differences (21 files).
• Syntax: 0 failures across 21 files.
• Focused tests (main, policy/quota routes, quota and usage services): 46 passed, identical before and after.
• Full suite: 160 passed before and after.
• git diff --check is clean and only files under tests/ changed.

Notes for reviewers

• app/main.py runs Base.metadata.create_all at import against a MySQL host, so all test runs used a scratch pytest plugin that blocks non-local socket connections and counts attempts: 0 attempts, and no live infrastructure was touched.
• The repo ships Cython-compiled .so files next to their .py sources; the *_source tests load the .py files directly via tests/_srcload.py, and the docstrings describe that source behavior.
• The policy and quota routes decide GPU and DGX access from the roles list in the request body; the docstrings say "roles supplied in the request" and make no authentication claim.
• test_queries_correct_user_id only asserts that the UsageRecord table is queried, not the user id. It is documented by what it asserts and left unchanged.

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