test : added unit tests for main.py exception handlers and redirect endpoints

[tmdeveloper007]

Closes #1725.

Summary of What Has Been Done:
Extracted exception handlers and redirect endpoints from main.py into a standalone import-safe module (backend/secuscan/main_exception_handlers.py) so they can be unit tested without the heavy app initialization chain. The main.py now delegates to the extracted handlers. Added comprehensive unit tests for all handlers and redirects.

Changes Made:

• New file: backend/secuscan/main_exception_handlers.py - standalone exception handlers and redirects
• New file: testing/backend/unit/test_main.py - 22 unit tests covering all handlers
• Modified: backend/secuscan/main.py - delegates to extracted handlers (no behavior change)

Impact it Made:

• Coverage: adds tests for a module that was completely untested
• Reliability: ensures consistent error responses across all error types
• Extensibility: extracted handlers are now independently testable

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

[utksh1]

Closing: this PR is part of the same mass template-generated low-value test batch pattern. The PRs use repetitive boilerplate bodies, copy-pasted structures, and in several cases manufacture tiny helpers or duplicate existing coverage rather than contributing one focused, meaningful improvement. Please submit a smaller, genuinely curated PR if you want to work on one concrete test gap.