test : add unit tests for _unshare_net_supported in parser_sandbox

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the _unshare_net_supported function in backend/secuscan/parser_sandbox.py. This function probes whether the unshare --user --net system call works on the host, and caches the result to avoid repeated probes.

The function has several distinct code paths that should each be tested:

1. Non-Linux platform returns False immediately
2. unshare binary not found returns False
3. unshare binary found but probe fails (non-zero exit) returns False
4. unshare binary found and probe succeeds (exit 0) returns True

Changes that Need to be Made:
Create testing/backend/unit/test_parser_sandbox_unshare_supported.py:

• Test non-Linux path by patching platform.system() to return "Darwin"
• Test unshare binary not found by patching shutil.which to return None
• Test probe failure by patching subprocess.run to return a non-zero returncode
• Test probe success by patching subprocess.run to return zero returncode
• The function uses module-level caching (_unshare_capability_checked, _unshare_available): each test case needs to reset these globals before and after to ensure test isolation
• Import the real production function directly: from backend.secuscan.parser_sandbox import _unshare_net_supported
• Run with python3 -m pytest testing/backend/unit/test_parser_sandbox_unshare_supported.py -v --noconftest

Impact that it would Provide:

• Tests a code path (capability detection) that is currently completely untested
• Prevents regressions when refactoring the unshare support detection logic
• Ensures the function behaves correctly on different platforms and when the unshare binary is unavailable
• Uses standard unittest.mock patching, no heavy framework dependencies

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.