fix: resolve Pi LLM auth through modelRegistry

- Call ctx.modelRegistry.getApiKeyAndHeaders(model) to resolve auth
- Pass resolved apiKey and headers to complete() options
- Provides clear error message if auth fails ( directs user to /login )
- Update test mock to include ok: true on auth response
- No external fetch() — Pi handles all transport and auth

tests/pi-extension.test.ts

@@ -60,6 +60,7 @@ describe("pi-extension", () => {
 			},
 			modelRegistry: {
 				getApiKeyAndHeaders: vi.fn(async () => ({
+					ok: true,
 					apiKey: "test-key",
 					headers: {},
 				})),