feat(KB-061): add model filter search to model selectors

- Add search input with clear button and results count to ModelSelectorTab
- Extract filterModels utility for shared filtering logic across components
- Apply filtering to SettingsModal model section
- Add CSS styles for filter UI components
- Add comprehensive tests for ModelSelectorTab and SettingsModal filtering

.changeset/add-model-filter-search.md

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+"@dustinbyrne/kb": patch
+---
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+Add type-to-filter search to model selection dropdowns. Users can now quickly find AI models by typing to filter by provider, model ID, or name.