Add Vector Search plugin

[adamgurary]

Summary

• Adds @databricks/appkit-vector-search — a plugin that gives Databricks Apps built with AppKit instant vector search query capabilities
• Ships backend (Express routes, VS REST API client, service principal + OBO auth) and frontend (React hook, styled components with Radix UI)
• Developer experience target: ~45 lines for a full search page with search box, results, filters, and keyword highlighting
• 82 tests included; validated against real VS index on dogfood

What's included

Backend plugin (src/plugin/):

• VectorSearchPlugin.ts — plugin class with lifecycle, config, route injection
• VectorSearchClient.ts — REST API client for VS endpoints
• routes.ts — Express route handlers
• auth.ts — service principal default, OBO opt-in per index

Frontend UI (src/ui/):

• useVectorSearch React hook
• SearchBox, SearchResults, SearchResultCard, SearchLoadMore components

Design decisions

Decision	Choice	Rationale
Package structure	Single package (backend + UI)	Shared types, single dependency, matches Lakebase plugin pattern
Default search mode	Hybrid (ANN + keyword)	Best out-of-the-box quality
Reranking	Off by default, opt-in per index	Adds latency; too slow for interactive search by default
Auth	Service principal default, OBO opt-in	Simple default, secure option when needed

Test plan

• Review plugin structure against existing AppKit plugin patterns (Lakebase)
• Run test suite (vitest run in packages/vector-search/)
• Validate against live VS index on dogfood
• Review API surface and types