ui: Refactor models store, MCP service, and gate logs behind VITE_DEBUG (#23236)
* refactor: Scope console logs to `DEV` + `VITE_DEBUG` env vars
* refactor: skip MCP proxy probe when no server requires it
* refactor: suppress expected disconnect errors during MCP client shutdown
* refactor: Deduplicate requests
* refactor: deduplicate model fetching across ROUTER and MODEL modes
* refactor: Clean up models logic
* chore: Add `.env.example` file
* refactor: replace client-side CORS proxy probe with server status flag
* refactor: Post-review fixes
* test: add vitest client setup with API fetch mocks