feat: auto-detect models from custom providers

Add 'Detect Models' button to custom provider forms that calls the
provider's /models endpoint to discover available models automatically.

- Supports OpenAI-compatible, Anthropic-compatible, and Google
  Generative AI providers
- Auto-fills context window and max tokens from provider response
- Filters out embedding, reranking, and non-text models
- Removes empty default model rows after detection
- Added comprehensive backend and frontend test coverage

packages/dashboard/app/components/__tests__/CustomProvidersSection.test.tsx

@@ -20,6 +20,7 @@ vi.mock("lucide-react", () => ({
   Loader2: ({ className }: { className?: string }) => <svg data-testid="icon-loader" className={className} />,
   Pencil: () => <svg data-testid="icon-pencil" />,
   Plus: () => <svg data-testid="icon-plus" />,
+  Search: () => <svg data-testid="icon-search" />,
   Trash2: () => <svg data-testid="icon-trash" />,
 }));