chore: add diagnosticable properties to channel_preview_theme.dart

packages/stream_chat_flutter/lib/src/theme/channel_preview_theme.dart

@@ -1,8 +1,9 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:stream_chat_flutter/src/theme/avatar_theme.dart';
 
 /// Theme for channel preview
-class ChannelPreviewTheme {
+class ChannelPreviewTheme with Diagnosticable {
   /// Constructor for creating [ChannelPreviewTheme]
   const ChannelPreviewTheme({
     this.titleStyle,
@@ -62,4 +63,15 @@ class ChannelPreviewTheme {
       unreadCounterColor: other.unreadCounterColor,
     );
   }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties
+      ..add(DiagnosticsProperty('titleStyle', titleStyle))
+      ..add(DiagnosticsProperty('subtitleStyle', subtitleStyle))
+      ..add(DiagnosticsProperty('lastMessageAtStyle', lastMessageAt))
+      ..add(DiagnosticsProperty('avatarTheme', avatarTheme))
+      ..add(ColorProperty('unreadCounterColor', unreadCounterColor));
+  }
 }