test : add unit tests for _check_field helper in validation

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the _check_field helper function in backend/secuscan/validation.py. This private function validates individual input field values (strings and lists) against length limits. It is the core of the validate_task_start_payload function and has distinct logic paths for string vs list values.

Changes that Need to be Made:

• Add a new file testing/backend/unit/test_validation_check_field.py
• Import _check_field and settings from backend.secuscan.validation
• For string values:
  • Test string at exactly max length passes
  • Test string exceeding max length returns 400 with appropriate message
  • Test string well under limit passes
• For list values:
  • Test list at exactly max array length passes
  • Test list exceeding max array length returns 400
  • Test list with items exceeding max field length returns 400
  • Test list with all items within limits passes
• For non-string/non-list values:
  • Test returns (True, 0, '') for unexpected types (pass-through)
• Test that error messages do not echo back field values
• Test with edge case: empty string ("") passes
• Test with edge case: empty list ([]) passes

Impact that it would Provide:

• Provides granular coverage for the field-level validation logic
• Ensures string length and array length limits are enforced correctly
• Guards against regression as settings limits are adjusted

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