feat(plugins): add search refinement to plugin catalog (#600)

[annukumar123]

Summary

Adds real-time search refinement filtering to the plugin catalog for issue #600.

Key Changes

• Search Input: Added a search refinement field directly above the plugin selector options.
• Real-Time Filter: Implemented case-insensitive client-side filtering by plugin name and description.
• Empty State: Renders a fallback message ("No matching plugins found.") when no plugins match the query.
• Unit Tests: Added test suite covering real-time filtering and empty match states.

Closes #600

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