Resolve Client.Go-002, -003 code-review findings

Client.Go-002: the Events/EventsAfter compatibility path silently dropped
events when the 16-slot results channel filled — it cancelled the stream and
closed the channel with no error delivered. sendEventResult now evicts an
old buffered event and delivers a terminal EventResult carrying the new
exported ErrEventBufferOverflow before close, so the overflow is observable.

Client.Go-003: parseInt32List panicked on a malformed -item-handles token,
crashing the CLI with a stack trace. It now returns an error that
runUnsubscribeBulk propagates, exiting 2 with a clean message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/go/mxgateway/session.go

@@ -490,7 +490,7 @@ func ensureBulkSize(name string, length int) error {
 
 func sendEventResult(
 	ctx context.Context,
-	results chan<- EventResult,
+	results chan EventResult,
 	result EventResult,
 	cancelWhenBufferFull bool,
 	cancel context.CancelFunc,
@@ -502,7 +502,12 @@ func sendEventResult(
 		case <-ctx.Done():
 			return false
 		default:
+			// The bounded compatibility buffer is full. Cancel the stream and
+			// deliver an explicit terminal overflow error so a slow consumer
+			// can tell dropped events apart from a normal end-of-stream,
+			// rather than seeing the channel close silently.
 			cancel()
+			deliverTerminalResult(results, EventResult{Err: ErrEventBufferOverflow})
 			return false
 		}
 	}
@@ -515,6 +520,25 @@ func sendEventResult(
 	}
 }
 
+// deliverTerminalResult places result on a full buffered channel by evicting
+// one of the oldest buffered events to make room. The caller closes results
+// afterwards, so the terminal result becomes the consumer's last item.
+func deliverTerminalResult(results chan EventResult, result EventResult) {
+	for {
+		select {
+		case results <- result:
+			return
+		default:
+		}
+		select {
+		case <-results:
+		default:
+			// Another receiver drained the channel between the send and
+			// receive attempts; retry the send.
+		}
+	}
+}
+
 func (s *Session) invokeCommand(ctx context.Context, command *MxCommand) (*MxCommandReply, error) {
 	return s.client.Invoke(ctx, &pb.MxCommandRequest{
 		SessionId:           s.ID(),