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

tests/unit/pi-worker-pool.test.ts

@@ -52,6 +52,10 @@ class FakeWorker implements PiConversationWorker {
     return { type: 'response' as const, id: 'fake', success: true };
   }
 
+  async send(command: PiRpcCommand): Promise<void> {
+    this.requests.push(command);
+  }
+
   subscribe(listener: (event: PiRpcEvent) => void): () => void {
     this.eventListeners.add(listener);
     return () => this.eventListeners.delete(listener);