feat: full OpenAPI 3.1 spec for the billing, usage, and developer routes

[emarc99]

• Closes: Generate and serve an OpenAPI 3.1 spec for the billing, usage, and developer routes #315

Description

This PR implements and serves a machine-readable OpenAPI 3.1 specification covering the billing, usage, and developer routes, and introduces an automated coverage and schema validation test suite.

Key Changes

1. OpenAPI 3.1 Contract (docs/openapi.json): Authored a complete, valid OpenAPI 3.1 specification detailing the /api/billing/deduct, /api/usage, /api/apis, and /api/developers/revenue endpoints, including exact request/response schemas and standardized error shapes (ErrorResponse).
2. Endpoint Serving (src/routes/index.ts): Mounted GET /api/openapi.json to dynamically serve the contract. Replaced the fileURLToPath ES-module logic with process.cwd() to prevent CommonJS/Jest transpilation issues.
3. Automated Spec Validation & Route Coverage (src/app.test.ts): Added tests verifying:
   • The OpenAPI route returns a valid 3.1 spec.
   • Target endpoints are fully described with secure error shapes.
   • Auto-detection of registered Express routes to flag any undocumented endpoints in CI.
4. Pre-existing Test Fixes:
   • Timezone Offset: Standardized week boundary expectations using UTC dates to prevent regional timezone assertion failures.
   • Mock Database Crash: Passed standard api mock parameters to the isolation route checker to prevent database query failures in better-sqlite3.
   • Route Classifications: Properly updated route protection expectations for /api/usage (requires auth) and /api/openapi.json (public).

Verification Results

• Ran npm test -- src/app.test.ts to execute all 58 suite tests.
• Result: All 58 tests passed successfully.

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[emarc99]

@greatest0fallt1me build errors are unrelated to my changes.