test(backend): cover CacheClient delete_prefix with expired and empty matches

[utksh1]

Summary

Add focused tests for delete_prefix when keys are already expired or no keys match, including consistency of the TTL and access-order maps.

Scope

• Suggested file: backend/secuscan/cache.py
• Add or extend the matching backend unit or integration test.
• Keep the change focused on the documented edge case.

Acceptance criteria

• The regression case is reproducible in an automated test.
• The expected safe behavior is asserted explicitly.
• Existing behavior remains covered.
• The focused test passes locally.

Verification

Run the affected pytest file and any directly related suite.