actions: Omit 'Mark as unread' for messages in unsubscribed streams.

Previously, the 'Mark as unread' button was displayed for any read message..

This pr adds a change that adds a condition to only show the button for  stream messages where a subscription exists in . a new test is added to verify this new behavior as requested.

Author: loveucifer

test/widgets/action_sheet_test.dart

@@ -1340,7 +1340,39 @@ void main() {
 
         check(find.byIcon(Icons.mark_chat_unread_outlined).evaluate()).single;
       });
+      testWidgets('not visible if message is in unsubscribed stream', (tester) async {
+        addTearDown(testBinding.reset);
+
+        final stream = eg.stream();
+        final readMessage = eg.streamMessage(stream: stream, flags: [MessageFlag.read]);
+        final narrow = TopicNarrow.ofMessage(readMessage);
+
+        // Manually set up the store, but without subscribing to the stream.
+        await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+        store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+        connection = store.connection as FakeApiConnection;
+        await store.addUser(eg.selfUser);
+        await store.addUser(eg.user(userId: readMessage.senderId));
+        await store.addStream(stream);
+        // Do NOT add the subscription.
+
+        // Prepare the API response for the message list.
+        connection.prepare(json: eg.newestGetMessagesResult(
+          foundOldest: true, messages: [readMessage]).toJson());
+
+        // Build the message list page.
+        await tester.pumpWidget(TestZulipApp(accountId: eg.selfAccount.id,
+          child: MessageListPage(initNarrow: narrow)));
+        await tester.pumpAndSettle();
+
+        // Open the action sheet for the message.
+        await tester.longPress(find.byType(MessageContent), warnIfMissed: false);
+        await tester.pump(const Duration(milliseconds: 250)); // animation
+        check(find.byType(BottomSheet)).findsOne();
 
+        // Now, verify the button is not present.
+        check(find.byIcon(Icons.mark_chat_unread_outlined).evaluate()).isEmpty();
+      });
       group('onPressed', () {
         testWidgets('smoke test', (tester) async {
           final message = eg.streamMessage(flags: [MessageFlag.read]);
@@ -1547,7 +1579,6 @@ void main() {
           of: find.byWidget(snackbar.content),
           matching: find.text(zulipLocalizations.successMessageTextCopied)));
       });
-
       testWidgets('request has an error', (tester) async {
         final message = eg.streamMessage();
         await setupToMessageActionSheet(tester, message: message, narrow: TopicNarrow.ofMessage(message));