test(backend): cover malformed remediation payload guardrails

[utksh1]

Summary

Add tests for malformed structured remediation inputs and verify the safety layer returns a controlled result.

Scope

• Suggested file: backend/secuscan/remediation.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.

[Srv99x]

Hi @utksh1 and mentors! I am a backend contributor focusing on Python and AI systems. I would love to write the regression tests for this issue under GSSoC '26. Could you please assign it to me?