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/cmd/mxgw-go/main.go

@@ -331,6 +331,11 @@ func runUnsubscribeBulk(ctx context.Context, args []string, stdout, stderr io.Wr
 		return errors.New("session-id and item-handles are required")
 	}
 
+	handles, err := parseInt32List(*itemHandles)
+	if err != nil {
+		return err
+	}
+
 	client, options, err := dialForCommand(ctx, common)
 	if err != nil {
 		return err
@@ -338,7 +343,7 @@ func runUnsubscribeBulk(ctx context.Context, args []string, stdout, stderr io.Wr
 	defer client.Close()
 
 	session := mxgateway.NewSessionForID(client, *sessionID)
-	results, err := session.UnsubscribeBulk(ctx, int32(*serverHandle), parseInt32List(*itemHandles))
+	results, err := session.UnsubscribeBulk(ctx, int32(*serverHandle), handles)
 	return writeBulkOutput(stdout, *jsonOutput, "unsubscribe-bulk", options, results, err)
 }
 
@@ -514,7 +519,7 @@ func parseStringList(value string) []string {
 	return items
 }
 
-func parseInt32List(value string) []int32 {
+func parseInt32List(value string) ([]int32, error) {
 	parts := strings.Split(value, ",")
 	items := make([]int32, 0, len(parts))
 	for _, part := range parts {
@@ -524,11 +529,11 @@ func parseInt32List(value string) []int32 {
 		}
 		parsed, err := strconv.ParseInt(item, 10, 32)
 		if err != nil {
-			panic(err)
+			return nil, fmt.Errorf("invalid item handle %q: %w", item, err)
 		}
 		items = append(items, int32(parsed))
 	}
-	return items
+	return items, nil
 }
 
 func bindCommonFlags(flags *flag.FlagSet) *commonOptions {