test : add unit tests for main.py exception handlers

[tmdeveloper007]

Summary of What Needs to be Done:
backend/secuscan/main.py contains several exception handler functions and redirect functions that have no test coverage. No test file exists for main.py. The testable functions include:

• rate_limit_exceeded_handler: handles RateLimitExceeded exceptions
• generic_rate_limit_handler: handles generic exceptions as rate limit
• custom_http_exception_handler: handles StarletteHTTPException
• custom_validation_exception_handler: handles RequestValidationError
• custom_unhandled_exception_handler: handles unexpected exceptions
• redirect_api_docs / redirect_api_redoc / redirect_api_openapi: redirect to root
• health_check: returns basic health status
• root: returns API root information

Changes that Need to be Made:
Create testing/backend/unit/test_main.py using FastAPI TestClient. Test each handler by:

• Calling the handler with a mock Request and appropriate Exception
• Asserting the response status code and body content

Impact that it would Provide:

• Reliability: ensures error responses are correctly formatted for all error types
• Coverage: adds tests for a module that is currently completely untested
• Prevents regressions in error handling behavior

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.