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/regressions/5596-missing-theme-export.test.ts

@@ -7,10 +7,10 @@ import { afterEach, describe, expect, it } from "vitest";
 import { AgentSession } from "../../../src/core/agent-session.ts";
 import { AuthStorage } from "../../../src/core/auth-storage.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 { SettingsManager } from "../../../src/core/settings-manager.ts";
 import { initTheme } from "../../../src/modes/interactive/theme/theme.ts";
+import { createInMemoryModelRegistry, getModelRuntime } from "../../model-runtime-test-utils.ts";
 import { createTestResourceLoader } from "../../utilities.ts";
 
 describe("regression #5596: missing configured theme export", () => {
@@ -32,8 +32,8 @@ describe("regression #5596: missing configured theme export", () => {
 
 		const model = faux.getModel();
 		const authStorage = AuthStorage.inMemory();
-		authStorage.setRuntimeApiKey(model.provider, "faux-key");
-		const modelRegistry = ModelRegistry.inMemory(authStorage);
+		await authStorage.modify(model.provider, async () => ({ type: "api_key", key: "faux-key" }));
+		const modelRegistry = await createInMemoryModelRegistry(authStorage);
 		modelRegistry.registerProvider(model.provider, {
 			baseUrl: model.baseUrl,
 			apiKey: "faux-key",
@@ -67,7 +67,7 @@ describe("regression #5596: missing configured theme export", () => {
 			sessionManager,
 			settingsManager,
 			cwd: tempDir,
-			modelRegistry,
+			modelRuntime: getModelRuntime(modelRegistry),
 			resourceLoader: createTestResourceLoader(),
 		});
 		cleanups.push(() => {