test(backend): cover execution context normalization for malformed stored JSON

[utksh1]

Summary

Add tests for non-object, malformed, and null-like stored execution-context payloads so they normalize to the documented safe default.

Scope

• Suggested file: backend/secuscan/execution_context.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.

[AI-Explorer786]

/assign