test : added vitest unit tests for requireAssessmentAccess middleware

[tmdeveloper007]

Closes #1900.

Summary of What Has Been Done:
Added 8 vitest unit tests for the requireAssessmentAccess middleware. Tests cover invalid assessment IDs, unauthorized access, assessment-not-found 404s, IDOR protection (returns 404 not 403 to avoid existence leakage), authorized access with req.assessment attachment, and unexpected error handling.

Changes Made:

• server/middleware/requireAssessmentAccess.test.ts (new file)

Impact it Made:

• Validates IDOR protection: unauthorized users get 404 (not 403) to prevent record existence leakage
• Confirms audit logging for both allowed and denied access attempts
• All 8 tests pass locally

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