Align unit tests with production modules

[dceoy]

Summary

• Mirror top-level and router production modules under tests/unit/.
• Add the constants module test and an automated bijection contract.
• Move CLI and application-wide prefix/OpenAPI checks to integration/contract scopes.
• Merge lifespan coverage into tests/unit/test_main.py while preserving coverage.

Verification

• pytest: 247 passed
• Coverage: 100% (configured threshold preserved)
• Ruff: passed
• Pyright: 0 errors
• Prettier, actionlint, yamllint, zizmor, and Checkov: passed

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[dceoy]

Review pass for 7826a14. Two actionable contract gaps were found and anchored inline.

[dceoy]

No new actionable findings were found in this pass;