test(backend): cover sandbox cleanup after container startup failure

[utksh1]

Summary

Add tests for a container start failure and verify temporary resources and task state are cleaned up.

Scope

• Suggested file: backend/secuscan/sandbox_executor.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.

[pratyushranjn]

Hi @utksh1 , I'd like to work on this issue as part of GSSoC 2026. Could you please assign it to me?