feat(api): BA-114 standardize API validation error responses with stable error code and field details

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-114 to standardize API validation error responses across all endpoints.

• Implements validationExceptionFactory and formatValidationErrors in app.controller.ts to transform class-validator ValidationError collections into a structured format with stable code: 'VALIDATION_FAILED'.
• Includes detailed per-field violation information (field, value, constraints) in the response payload.
• Wires validationExceptionFactory into the global ValidationPipe in main.ts.
• Adds comprehensive unit tests in app.controller.spec.ts covering validation error transformation, stable error code verification, and integration with ValidationPipe.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/main.ts
  • BackendAcademy/src/app.controller.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1027