test : added unit tests for validate_plugins_catalog script helpers

[tmdeveloper007]

Closes #2070.

Summary of What Has Been Done:

Added testing/backend/unit/test_validate_plugins_catalog.py with 11 unit tests covering the four helper functions in scripts/validate_plugins_catalog.py.

Changes Made:

• parse_plugins_md: tests for plugin entry extraction from markdown table, header row skipping, empty file handling
• extract_counts_from_catalog: tests for "At a Glance" count extraction (total, safe, intrusive, exploit)
• extract_category_counts_from_catalog: tests for category table parsing and stopping at next section header
• validate_catalog: tests for in-sync (True, empty issues), missing from catalog, extra in catalog, count mismatch cases

All tests use tempfile.TemporaryDirectory for fixture data. Tests pass with python3 -m pytest --noconftest.

Impact it Made:

• Validates the CI catalog validation script itself, preventing silent breakage
• Catches regressions in regex patterns for markdown table parsing
• Improves confidence when adding or renaming plugins

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

[utksh1]

Closing as superseded by #2079, whose branch already contains this commit. Please avoid stacked duplicate PRs; keep one focused branch per reviewable change.

[utksh1]

The catalog helper coverage is a reasonable direction, but the branch fails backend-unit and has no linked assigned issue. Please add a focused issue reference, remove redundant parser-format assertions where existing CI already covers them, and fix the suite before re-review.