feat(coding-agent): replace model registry with model runtime

Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.

packages/coding-agent/test/extensions-runner.test.ts

@@ -1,3 +1,4 @@
+import { createModelRegistry } from "./model-runtime-test-utils.ts";
 /**
  * Tests for ExtensionRunner - conflict detection, error handling, tool wrapping.
  */
@@ -16,7 +17,7 @@ import type {
 	ProviderConfig,
 } from "../src/core/extensions/types.ts";
 import { KeybindingsManager, type KeyId } from "../src/core/keybindings.ts";
-import { ModelRegistry } from "../src/core/model-registry.ts";
+import type { ModelRegistry } from "../src/core/model-registry.ts";
 import { SessionManager } from "../src/core/session-manager.ts";
 
 describe("ExtensionRunner", () => {
@@ -26,13 +27,13 @@ describe("ExtensionRunner", () => {
 	let modelRegistry: ModelRegistry;
 	const defaultKeybindings = new KeybindingsManager().getEffectiveConfig();
 
-	beforeEach(() => {
+	beforeEach(async () => {
 		tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-runner-test-"));
 		extensionsDir = path.join(tempDir, "extensions");
 		fs.mkdirSync(extensionsDir);
 		sessionManager = SessionManager.inMemory();
 		const authStorage = AuthStorage.create(path.join(tempDir, "auth.json"));
-		modelRegistry = ModelRegistry.create(authStorage);
+		modelRegistry = await createModelRegistry(authStorage);
 	});
 
 	afterEach(() => {
@@ -817,7 +818,7 @@ describe("ExtensionRunner", () => {
 	});
 
 	describe("provider registration", () => {
-		it("bindCore ignores invalid queued registrations and reports extension error", () => {
+		it("bindCore ignores invalid queued registrations and reports extension error", async () => {
 			const runtime = createExtensionRuntime();
 			runtime.registerProvider(
 				"broken-provider",
@@ -837,7 +838,7 @@ describe("ExtensionRunner", () => {
 			expect(errors).toEqual([
 				'/tmp/broken-extension.ts: Provider broken-provider: "api" is required when registering streamSimple.',
 			]);
-			expect(() => modelRegistry.refresh()).not.toThrow();
+			await expect(modelRegistry.refresh()).resolves.toBeUndefined();
 		});
 
 		it("pre-bind unregister removes all queued registrations for a provider", () => {