Fix: Plugin Cache Invalidation (#800)

[diksha78dev]

Description

This PR refactors the cache invalidation logic by moving it into cache.py to prevent circular dependencies. It introduces invalidate_plugin_caches() to ensure that plugin and dashboard summary caches are correctly cleared whenever plugin states change. Additionally, it adds a try-except RuntimeError block around cache retrievals to gracefully handle scenarios where the cache is not initialized (such as in certain unit testing environments).

Related Issues

Fixes #800

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Updated testing/backend/test_cache_invalidation.py to verify both invalidate_view_cache and the new invalidate_plugin_caches.
• Added a specific test test_load_plugins_invalidates_cache to mock and verify that plugin cache invalidation is correctly triggered during plugin loading.
• Ensured backwards compatibility by verifying invalidate_view_cache remains importable from routes.py.

To reproduce the test results, run:
pytest testing/backend/test_cache_invalidation.py

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]

Focused change with green checks; approving.