kadm: fix ListTopics filtering internal topics when explicitly requested

ListTopics was unconditionally filtering internal topics even when
specific topic names were provided. This violated the documented
behavior that internal topics should be included when explicitly
requested by name.

This fix only applies FilterInternal when listing all topics (no
specific topics requested), allowing explicitly named internal topics
like __consumer_offsets to be returned as documented.

Author: weeco

pkg/kadm/topics.go

@@ -26,7 +26,11 @@ func (cl *Client) ListTopics(
 	if err != nil {
 		return nil, err
 	}
-	t.FilterInternal()
+	// Only filter internal topics when listing all topics (no specific topics requested).
+	// If specific topics are named, include them even if they are internal.
+	if len(topics) == 0 {
+		t.FilterInternal()
+	}
 	return t, nil
 }