Fix panic on send to closed event channel in Go SDK

Protect dispatchEvent with a recover guard so that a notification
arriving after Disconnect does not crash the process. Also wrap the
channel close in sync.Once so Disconnect is safe to call more than once.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

go/session.go

@@ -67,7 +67,8 @@ type Session struct {
 
 	// eventCh serializes user event handler dispatch. dispatchEvent enqueues;
 	// a single goroutine (processEvents) dequeues and invokes handlers in FIFO order.
-	eventCh chan SessionEvent
+	eventCh  chan SessionEvent
+	closeOnce sync.Once // guards eventCh close so Disconnect is safe to call more than once
 
 	// RPC provides typed session-scoped RPC methods.
 	RPC *rpc.SessionRpc
@@ -451,6 +452,11 @@ func (s *Session) handleHooksInvoke(hookType string, rawInput json.RawMessage) (
 // serial, FIFO dispatch without blocking the read loop.
 func (s *Session) dispatchEvent(event SessionEvent) {
 	go s.handleBroadcastEvent(event)
+
+	// Use a recover guard because eventCh may be closed by Disconnect.
+	// In Go, sending on a closed channel panics and there is no
+	// non-panicking send primitive.
+	defer func() { recover() }()
 	s.eventCh <- event
 }
 
@@ -657,7 +663,7 @@ func (s *Session) Disconnect() error {
 		return fmt.Errorf("failed to disconnect session: %w", err)
 	}
 
-	close(s.eventCh)
+	s.closeOnce.Do(func() { close(s.eventCh) })
 
 	// Clear handlers
 	s.handlerMutex.Lock()