test : added unit tests for _check_field helper in validation

[tmdeveloper007]

Closes #2337.

Summary of What Has Been Done:
Added 20 unit tests for the _check_field helper function in backend/secuscan/validation.py. This private function validates individual input field values against string length and array length limits, and is the core of the validate_task_start_payload function.

Changes Made:

• New file: testing/backend/unit/test_validation_check_field.py
• String value tests: at-max-length (pass), exceeding (reject), under-limit (pass), empty (pass)
• List value tests: at-max-array-length (pass), exceeding (reject), items over limit (reject)
• Nested dict tests: short values (pass), long string in nested dict (reject with dotted key), long list in nested dict (reject)
• Non-string/non-list pass-through (int, float, None, bool)
• Error message safety tests: verifies field names appear but values do not echo back

Impact it Made:

• 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.

[tmdeveloper007]

Hi, this PR has been waiting for review for a few days now. All CI checks are green. Could you please take a look when you have a moment? Happy to make any adjustments if needed.

[utksh1]

Reviewed the focused test change and its current checks are green. The PR still needs a current branch update before merge because it is behind main; I will not bypass the branch policy.