feat(api): BA-115 echo or generate request correlation IDs on error responses

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-115 to ensure all API error responses echo or generate a request correlation ID for consistent tracing and observability.

• Implements getOrGenerateCorrelationId to inspect incoming headers (x-correlation-id, x-request-id) or generate a secure UUID.
• Adds CorrelationIdExceptionFilter in app.controller.ts and binds it globally via app.useGlobalFilters(...) in main.ts.
• Attaches x-correlation-id to response headers and embeds correlationId, statusCode, message, and timestamp into error payloads.
• Adds comprehensive unit tests in app.controller.spec.ts testing header extraction, UUID generation fallback, and filter payload formatting across client and server exceptions.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/main.ts
  • BackendAcademy/src/app.controller.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1028