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/agent-session-tree-navigation.test.ts

@@ -15,8 +15,8 @@ import { API_KEY, createTestSession, type TestSessionContext } from "./utilities
 describe.skipIf(!API_KEY)("AgentSession tree navigation e2e", () => {
 	let ctx: TestSessionContext;
 
-	beforeEach(() => {
-		ctx = createTestSession({
+	beforeEach(async () => {
+		ctx = await createTestSession({
 			systemPrompt: "You are a helpful assistant. Reply with just a few words.",
 			settingsOverrides: { compaction: { keepRecentTokens: 1 } },
 		});
@@ -279,8 +279,8 @@ describe.skipIf(!API_KEY)("AgentSession tree navigation e2e", () => {
 describe.skipIf(!API_KEY)("AgentSession tree navigation - branch scenarios", () => {
 	let ctx: TestSessionContext;
 
-	beforeEach(() => {
-		ctx = createTestSession({
+	beforeEach(async () => {
+		ctx = await createTestSession({
 			systemPrompt: "You are a helpful assistant. Reply with just a few words.",
 		});
 	});