feat(api): BA-119 map unmatched routes to consistent shared not-found API error responses

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-119 to ensure unmatched routes return consistent, structured JSON not-found error responses conforming to the shared API format.

• Implements NotFoundExceptionFilter in app.module.ts to intercept NotFoundException and format responses with { statusCode: 404, error: 'Not Found', message, path, timestamp }.
• Binds NotFoundExceptionFilter as APP_FILTER in AppModule and globally in main.ts.
• Adds unit test suite in app.controller.spec.ts verifying unmatched route payloads, status code 404, path reporting, and fallback handling when request URL is absent.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/main.ts
  • BackendAcademy/src/app.module.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1032