test(backend): cover parser sandbox UTF-8 output size boundaries

[utksh1]

Summary

Add byte-boundary tests with multibyte UTF-8 output to verify truncation and failure messages remain safe.

Scope

• Suggested file: backend/secuscan/parser_sandbox.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.

[Rakshak05]

Hii @utksh1, I would like to work on this. Can you please assign it to me under GSSoC'26 contributor!?