test : added unit tests for plugin_health_dashboard script helpers

[tmdeveloper007]

Closes #2071.

Summary of What Has Been Done:

Added testing/backend/unit/test_plugin_health_dashboard.py with 14 unit tests covering the four helper functions in scripts/plugin_health_dashboard.py.

Changes Made:

• safe_relative_path: tests for normal paths, exact match, outside-base (returns absolute), deeply nested
• discover_plugins: tests for list return, missing parser detection, invalid JSON handling, name sorting, empty directory
• build_report: tests for summary counts (total/with-parser/without), category aggregation, empty input
• format_markdown: tests for summary section content, alphabetical category order, plugin table with parser status

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

Impact it Made:

• Prevents regressions in plugin health reporting
• Validates markdown output format for maintainer dashboards
• Ensures discover_plugins correctly categorizes 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Closing as superseded by #2079, whose branch already contains this commit.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]

This branch includes the unrelated catalog-test PR beneath it and fails backend-unit. Please rebase independently onto main, keep only plugin-health dashboard tests, and fix CI.