fix: handle invalid JSON in semgrep parser and add unit tests

[vedasingh00-rgb]

Description

Fixed the semgrep_scanner parser to handle invalid JSON gracefully instead of silently ignoring errors. Added unit tests to verify all three scenarios — valid JSON, invalid JSON, and mixed stdout output.

Related Issues

#1421

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added 3 unit tests in testing/test_semgrep_parser.py and ran them using pytest:
test_valid_json — verifies valid Semgrep JSON is parsed correctly
test_invalid_json — verifies invalid JSON does not crash the parser
test_mixed_stdout — verifies mixed stdout is handled gracefully

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.