fix(CLI-01): Go Events() emits terminal ErrSlowConsumer on overflow

Events()/EventsAfter() previously closed the channel silently when the
16-slot buffer overflowed, indistinguishable from a graceful server end.
Reserve one slot and emit a terminal EventResult wrapping the new exported
ErrSlowConsumer so overflow is always observable. The blocking
SubscribeEvents path (gRPC flow-controlled) is unchanged.

archreview: CLI-01 (P0). Verified: gofmt clean, go build ./..., go test ./... (+ -race) pass.

clients/go/mxgateway/client_session_test.go

@@ -147,6 +147,59 @@ func TestEventsAfterCancelsStreamWhenCompatibilityChannelIsAbandoned(t *testing.
 	}
 }
 
+func TestEventsSlowConsumerYieldsErrSlowConsumerBeforeClose(t *testing.T) {
+	fake := &fakeGatewayServer{
+		streamStarted:    make(chan struct{}),
+		streamDone:       make(chan struct{}),
+		streamEventCount: 64,
+	}
+	client, cleanup := newBufconnClient(t, fake)
+	defer cleanup()
+	session := NewSessionForID(client, "session-1")
+
+	events, err := session.EventsAfter(context.Background(), 0)
+	if err != nil {
+		t.Fatalf("EventsAfter() error = %v", err)
+	}
+	<-fake.streamStarted
+
+	// Do not drain the channel so the buffer overflows. The stream stops once
+	// the slow-consumer cancel fires on the producer side.
+	select {
+	case <-fake.streamDone:
+	case <-time.After(2 * time.Second):
+		t.Fatal("event stream did not stop after buffer overflow")
+	}
+
+	var last EventResult
+	gotResult := false
+	for {
+		select {
+		case res, ok := <-events:
+			if !ok {
+				if !gotResult {
+					t.Fatal("events channel closed without yielding any result")
+				}
+				if !errors.Is(last.Err, ErrSlowConsumer) {
+					t.Fatalf("final event result err = %v, want one wrapping ErrSlowConsumer", last.Err)
+				}
+				var gwErr *GatewayError
+				if !errors.As(last.Err, &gwErr) {
+					t.Fatalf("final event result err is %T, want *GatewayError", last.Err)
+				}
+				if gwErr.Op != "stream events" {
+					t.Fatalf("final event result err Op = %q, want %q", gwErr.Op, "stream events")
+				}
+				return
+			}
+			last = res
+			gotResult = true
+		case <-time.After(2 * time.Second):
+			t.Fatal("events channel did not close after slow-consumer termination")
+		}
+	}
+}
+
 func TestSessionHelpersBuildCommandsAndExposeRawReply(t *testing.T) {
 	fake := &fakeGatewayServer{
 		invokeReply: &pb.MxCommandReply{