test : add unit tests for parser_sandbox pure helper functions

[tmdeveloper007]

Summary of What Needs to be Done:
backend/secuscan/parser_sandbox.py contains two testable units: _sanitised_env (which returns a minimal environment dict with credentials stripped) and run_parser_in_sandbox (the main sandboxed execution function). Both currently lack dedicated unit tests.

Changes that Need to be Made:

• Add testing/backend/unit/test_parser_sandbox.py
• Test _sanitised_env: retains PATH, PYTHONPATH, HOME, TEMP; strips API keys and secrets
• Test run_parser_in_sandbox: parser.py not found raises ParserSandboxError; timeout path; oversized output path; non-zero exit code path; non-JSON output path; success path with mocked subprocess
• Mock subprocess.Popen to isolate tests from actual subprocess spawning
• Patch os.environ to verify credential stripping behavior

Impact that it would Provide:

• Ensures secrets are never leaked to plugin parser subprocesses
• Validates sandbox boundaries (timeout, output size, exit code handling)
• Enables CI without requiring real plugin parser files

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