test(backend): cover IPv6 targets with ports in executor command construction

[utksh1]

Summary

Add tests for bracketed IPv6 URLs and host-port targets to ensure command construction preserves the intended target.

Scope

• Suggested file: backend/secuscan/executor_target_helpers.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.