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/suite/harness.ts

@@ -1,3 +1,4 @@
+import { createInMemoryModelRegistry, getModelRuntime } from "../model-runtime-test-utils.ts";
 /**
  * Local test harness for the new coding-agent test suite.
  */
@@ -18,7 +19,6 @@ import { AgentSession, type AgentSessionEvent } from "../../src/core/agent-sessi
 import { AuthStorage } from "../../src/core/auth-storage.ts";
 import type { ExtensionRunner } from "../../src/core/extensions/index.ts";
 import { convertToLlm } from "../../src/core/messages.ts";
-import { ModelRegistry } from "../../src/core/model-registry.ts";
 import { SessionManager } from "../../src/core/session-manager.ts";
 import type { Settings } from "../../src/core/settings-manager.ts";
 import { SettingsManager } from "../../src/core/settings-manager.ts";
@@ -113,9 +113,9 @@ export async function createHarness(options: HarnessOptions = {}): Promise<Harne
 
 	const authStorage = AuthStorage.inMemory();
 	if (withConfiguredAuth) {
-		authStorage.setRuntimeApiKey(model.provider, "faux-key");
+		await authStorage.modify(model.provider, async () => ({ type: "api_key", key: "faux-key" }));
 	}
-	const modelRegistry = ModelRegistry.inMemory(authStorage);
+	const modelRegistry = await createInMemoryModelRegistry(authStorage);
 	if (withConfiguredAuth) {
 		modelRegistry.registerProvider(model.provider, {
 			baseUrl: model.baseUrl,
@@ -178,7 +178,7 @@ export async function createHarness(options: HarnessOptions = {}): Promise<Harne
 		sessionManager,
 		settingsManager,
 		cwd: tempDir,
-		modelRegistry,
+		modelRuntime: getModelRuntime(modelRegistry),
 		resourceLoader,
 		baseToolsOverride: toolMap,
 		initialActiveToolNames: options.initialActiveToolNames,