feat(api): BA-120 enforce strict global request validation rejecting unknown DTO properties

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-120 to enforce strict global request validation across the application.

• Configures ValidationPipe with whitelist: true, transform: true, and forbidNonWhitelisted: true to reject unknown DTO properties without stripping or silently accepting them.
• Provides APP_PIPE in AppModule so strict validation is globally enforced across all module boundaries and integration contexts.
• Adds comprehensive unit tests in app.controller.spec.ts verifying that payloads with unrecognized fields throw BadRequestException with specific property warnings, while valid DTO payloads pass smoothly.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/main.ts
  • BackendAcademy/src/app.module.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1033