docs: improve test suite documentation

[man4ish]

Summary

• Add module/class/test docstrings across all 12 test modules (documentation only, no executable changes)
• Security-sensitive tests (generic_sif_runner, PHI-safe logging, redaction) document the actual invariant under test rather than paraphrasing the function name
• Verified via AST-equivalence diffing against the pre-change source that no executable semantics changed

Test plan

• Syntax validation on all 13 test files (ast.parse)
• AST-equivalence check (docstring-stripped) for every modified file vs. starting HEAD — 0 executable differences
• Full test suite: 414 passed (matches pre-change baseline, run repeatedly to rule out flakiness)
• git diff --check clean; no production/config files touched

🤖 Generated with Claude Code