test: document xss_exploiter failure-mode parser behaviour

[mythri2405]

Description

This PR adds parser contract coverage for the xss_exploiter plugin to document and lock in its failure-mode behavior. It does not change plugin runtime behavior, only improves test-level documentation of expected outputs.

Specifically, it clarifies how the parser handles:

• Empty output (no findings)
• Blocked payload messages
• Benign failure messages

All cases are now explicitly covered through unit tests to ensure deterministic behavior and prevent regression.

Related Issues

Closes #858

Type of Change

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

How Has This Been Tested?

The following test suite was executed:
python -m pytest testing/backend/unit/test_xss_exploiter_plugin.py -v

Results:

• 7 tests passed
• No failures
• No runtime changes to plugin behavior

Covered scenarios:

• Empty output returns deterministic empty findings
• Blocked payload messages are preserved as informational findings
• Benign failure messages are preserved as informational findings
• Existing parser + executor behavior remains unchanged

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.

[utksh1]

Requesting changes. The test addition is small, but the current run has frontend-checks failing. Please rerun/fix CI so the PR is fully green before it can be merged.

[utksh1]

Still blocked after the latest update. The parser tests are small, but the PR now includes unrelated frontend/package-lock dependency churn. Please split the package-lock/audit update out and keep this PR focused on xss_exploiter parser behavior.