test : add unit tests for _sanitize_stderr in parser_sandbox

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the _sanitize_stderr() helper function in backend/secuscan/parser_sandbox.py. This function strips absolute file paths and Python line-number references from stderr output before logging, to prevent leaking internal topology in diagnostic logs.

The existing test file testing/backend/unit/test_parser_sandbox.py covers 27 tests and testing/backend/unit/test_parser_sandbox_helpers.py covers the environment sanitization helpers, but _sanitize_stderr is not imported or tested in either file.

Changes that Need to be Made:
Create a new file testing/backend/unit/test_parser_sandbox_stderr_sanitize.py:

• Import _sanitize_stderr from backend.secuscan.parser_sandbox
• Test that absolute Unix paths (e.g. /home/user/project/file.py) are replaced with [PATH]
• Test that Windows absolute paths (e.g. C:\Users\project\file.py) are replaced with [PATH]
• Test that Python line number references (e.g. "line 42", "Line 1") are replaced with [LINE]
• Test that the function is case-insensitive for line number matching
• Test that multiple occurrences are all replaced
• Test that text without paths or line numbers is returned unchanged
• Test that the output is truncated to the max_chars default (500) for very long inputs
• Test that max_chars parameter controls the truncation length
• Test that the order of replacement is path first, then line numbers

Impact that it would Provide:

• Prevents internal file paths and line numbers from leaking into operator-facing logs
• Ensures the sanitizer handles edge cases (no paths, multiple paths, mixed content)
• Validates the truncation behavior to prevent memory issues from huge stderr

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