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/client_session_test.go

@@ -117,7 +117,7 @@ func TestEventsAfterCancelsStreamWhenCompatibilityChannelIsAbandoned(t *testing.
 	fake := &fakeGatewayServer{
 		streamStarted:    make(chan struct{}),
 		streamDone:       make(chan struct{}),
-		streamEventCount: 64,
+		streamEventCount: 256,
 	}
 	client, cleanup := newBufconnClient(t, fake)
 	defer cleanup()
@@ -135,12 +135,25 @@ func TestEventsAfterCancelsStreamWhenCompatibilityChannelIsAbandoned(t *testing.
 		t.Fatal("compatibility event stream did not stop after result channel filled")
 	}
 
+	// A slow consumer that abandons the buffer must still receive an explicit
+	// terminal overflow error before the channel closes, so it can tell
+	// "events dropped" apart from "stream ended normally".
+	var sawOverflow bool
 	for {
 		select {
-		case _, ok := <-events:
+		case result, ok := <-events:
 			if !ok {
+				if !sawOverflow {
+					t.Fatal("compatibility event channel closed without an ErrEventBufferOverflow result")
+				}
 				return
 			}
+			if result.Err != nil {
+				if !errors.Is(result.Err, ErrEventBufferOverflow) {
+					t.Fatalf("terminal result error = %v, want ErrEventBufferOverflow", result.Err)
+				}
+				sawOverflow = true
+			}
 		case <-time.After(2 * time.Second):
 			t.Fatal("compatibility event channel did not close")
 		}