test(task): stubbed setIrcWakeTurnObserver on executor session fakes

- The IRC wake monitor from PR #7108 calls the observer on every kept-alive
  subagent finalize; fakes across the executor suites predate the method
  and threw during finalization (69 failures).

packages/coding-agent/test/task/executor-prewalk.test.ts

@@ -74,6 +74,7 @@ function yieldEmittingSession(
 		abort: async () => {},
 		dispose: async () => {},
 	};
+	session.setIrcWakeTurnObserver = () => {};
 	return session as unknown as AgentSession;
 }