test: add tests for parser_sandbox._unshare_net_supported (closes #2444)

[tmdeveloper007]

Summary

Adds unit tests for backend.secuscan.parser_sandbox._unshare_net_supported,a module-level helper that probes for unshare(1) network-namespace support.

Changes

• New test file: testing/backend/unit/test_parser_sandbox_unshare_supported.py
• 10 tests covering: non-Linux fallthrough, binary-not-found path,
  successful probe, probe failure, subprocess exceptions, and caching
• Bug fix: early-return paths in _unshare_net_supported were not setting
  _unshare_available = False before returning, causing cached None
  on subsequent calls (fixed in backend/secuscan/parser_sandbox.py)

Testing

• All 10 tests pass with python -m pytest testing/backend/unit/test_parser_sandbox_unshare_supported.py --noconftest -v

[utksh1]

The code change is reviewed and the backend/lint checks are green. Merge remains blocked by the required frontend audit, which reports the pre-existing high-severity undici vulnerability; I am not bypassing that security gate.

[utksh1]

@tmdeveloper007 Please rebase this PR onto the latest main branch (2f82738) to resolve the merge conflict.