docs: improve test suite documentation

[man4ish]

Summary

• Add module, class, and test-function docstrings across all 12 unit-test modules in tests/, matching the documentation standard already established in omnibioai-workbench and omnibioai-api-gateway
• Every test_* function and Test* class now states the specific behavior or invariant it verifies; security-sensitive suites (delegated auth, tenant/org run ownership, tool invocation, request validation) document the exact contract each test proves
• Documentation only — no executable test behavior changed, verified via AST-equivalence diff against main for every modified file (0 executable differences)

Test plan

• py_compile on all modified files — clean
• AST-equivalence check (docstring-normalized) for every modified file against main — 0 executable differences
• Full pytest suite before and after — 386 passed in both runs, identical count
• git diff --check — no whitespace errors
• Coverage inventory — 12/12 modules documented, 12/12 Developer attribution, 65/65 Test* classes documented, 382/382 test_* functions documented

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