Message_list: Make topic headers case-insensitive

This change in the commit makes the message list's topic headers match that behavior, so messages whose topics differ only in case (like "missing string" and "Missing string") share a single header.  This brings the behavior in line with Zulip web. (Added the relevant test case)

Author: shivanshsharma13

lib/model/message_list.dart

@@ -321,7 +321,7 @@ mixin _MessageSequence {
 bool haveSameRecipient(Message prevMessage, Message message) {
   if (prevMessage is StreamMessage && message is StreamMessage) {
     if (prevMessage.streamId != message.streamId) return false;
-    if (prevMessage.topic != message.topic) return false;
+    if (prevMessage.topic.toLowerCase() != message.topic.toLowerCase()) return false;
   } else if (prevMessage is DmMessage && message is DmMessage) {
     if (!_equalIdSequences(prevMessage.allRecipientIds, message.allRecipientIds)) {
       return false;

test/model/message_list_test.dart

@@ -1703,6 +1703,23 @@ void main() {
         }
       }
     });
+    test('stream messages match with case-insensitive topics', () {
+      final stream = eg.stream();
+      final messageLower = eg.streamMessage(
+          stream: stream,
+          topic: 'hello world');
+      final messageUpper = eg.streamMessage(
+          stream: stream,
+          topic: 'Hello World');
+      final messageDiff = eg.streamMessage(
+          stream: stream,
+          topic: 'different');
+
+      check(haveSameRecipient(messageLower, messageUpper)).isTrue();
+      check(haveSameRecipient(messageUpper, messageLower)).isTrue();
+      check(haveSameRecipient(messageLower, messageDiff)).isFalse();
+      check(haveSameRecipient(messageUpper, messageDiff)).isFalse();
+    });
   });
 
   test('messagesSameDay', () {