feat(coding-runtime): add managed Pi extension host

tests/unit/pi-conversation-runtime.test.ts

@@ -77,6 +77,10 @@ class RuntimeFakeWorker implements PiConversationWorker {
     };
   }
 
+  async send(command: PiRpcCommand): Promise<void> {
+    this.requests.push(structuredClone(command));
+  }
+
   setSessionData(input: { state?: unknown; entries?: unknown; stats?: unknown }): void {
     if (input.state !== undefined) this.stateData = structuredClone(input.state);
     if (input.entries !== undefined) this.entriesData = structuredClone(input.entries);
@@ -240,6 +244,25 @@ describe('Pi Conversation runtime', () => {
       status: 'complete',
     }));
     expect(JSON.stringify(streamed.nodes)).toContain('Implemented');
+    workers.get(left.id)!.emit({
+      type: 'extension_ui_request',
+      id: 'question-runtime',
+      method: 'select',
+      title: 'Choose implementation',
+      options: ['Small seam', 'Large seam'],
+    });
+    await expect.poll(async () => (await runtime.getSnapshot(left.id)).pendingInteractions)
+      .toContainEqual(expect.objectContaining({ id: 'question-runtime', status: 'pending' }));
+    await runtime.respondInteraction(left.id, {
+      interactionId: 'question-runtime',
+      optionId: 'question-runtime:option:0',
+    });
+    expect(workers.get(left.id)!.requests.at(-1)).toEqual({
+      type: 'extension_ui_response', id: 'question-runtime', value: 'Small seam',
+    });
+    expect((await runtime.getSnapshot(left.id)).pendingInteractions).toContainEqual(
+      expect.objectContaining({ id: 'question-runtime', status: 'answered' }),
+    );
     const durable = {
       state: {
         sessionId: `session-${left.id}`,