test: add unit tests for _validate_lengths field length guard

[aaniya22]

Description

Adds unit tests for the _validate_lengths helper in backend/secuscan/routes.py, which enforces maximum field lengths (255 chars for name, 2000 chars for description/notes) before data reaches the database. Covers valid/boundary/over-limit cases for name, description, and notes fields, plus custom resource_type substitution and None handling for optional fields.

Related Issues

Closes #2307

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Ran python -m pytest testing/backend/unit/test_routes_validate_lengths.py -v locally — all 9 tests pass. Coverage includes:

• Valid name/description/notes under their respective limits
• Exact-boundary values (255 chars for name, 2000 for description/notes) — confirmed to pass
• Over-limit values for name, description, and notes — confirmed to raise HTTPException with status 400 and the correct detail message
• Custom resource_type correctly substituted into the error message
• None values for all optional fields — confirmed to pass without raising

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

The code change is reviewed and the backend/lint checks are green. Merge remains blocked by the required frontend audit, which reports the pre-existing high-severity undici vulnerability; I am not bypassing that security gate.

[utksh1]

Reviewed the current head, including the undici lockfile security update. The focused validation tests pass, all required checks are green, and the branch is clean for squash merge.