feat(agentcompat): add scoped IO stream capabilities

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

service/rpc/io_stream_state_agentcompat.go

@@ -0,0 +1,11 @@
+//go:build agentcompat
+
+package rpc
+
+// SetIOStreamStateWaitObserverForAgentcompat installs a deterministic harness
+// seam for observing that a waiter captured its notification channel.
+func (s *NezhaHandler) SetIOStreamStateWaitObserverForAgentcompat(observer func()) {
+	s.ioStreamMutex.Lock()
+	defer s.ioStreamMutex.Unlock()
+	s.ioStreamWaitLockedHook = observer
+}