test: Use TransitionDurationObserver in compose_box_test

Added TransitionDurationObserver initialization inside prepareComposeBox
setup,every test that calls prepareComposeBox now automatically attaches an
observer to the widget tree.Replaced hardcoded transition delays with dynamic duration handling
via transitionDurationObserver.pumpPastTransition(tester). This ensures test
stability when transition timings change.

Author: MdSaifAliMolla

test/widgets/compose_box_test.dart

@@ -50,6 +50,7 @@ void main() {
   late PerAccountStore store;
   late FakeApiConnection connection;
   late ComposeBoxState state;
+  late TransitionDurationObserver transitionDurationObserver;
 
   // Caution: when testing edit-message UI, this will often be stale;
   // read state.controller instead.
@@ -96,6 +97,7 @@ void main() {
     store = await testBinding.globalStore.perAccount(selfAccount.id);
 
     connection = store.connection as FakeApiConnection;
+    transitionDurationObserver = TransitionDurationObserver();
 
     connection.prepare(json:
       eg.newestGetMessagesResult(foundOldest: true, messages: messages).toJson());
@@ -104,6 +106,7 @@ void main() {
       connection.prepare(json: GetStreamTopicsResult(topics: []).toJson());
     }
     await tester.pumpWidget(TestZulipApp(accountId: selfAccount.id,
+      navigatorObservers: [transitionDurationObserver],
       child: MessageListPage(initNarrow: narrow)));
     await tester.pumpAndSettle();
     connection.takeRequests();
@@ -1719,7 +1722,7 @@ void main() {
     await tester.longPress(find.byWidgetPredicate((widget) =>
       widget is MessageWithPossibleSender && widget.item.message.id == messageId));
     // sheet appears onscreen; default duration of bottom-sheet enter animation
-    await tester.pump(const Duration(milliseconds: 250));
+    await transitionDurationObserver.pumpPastTransition(tester);
     final findEditButton = find.descendant(
       of: find.byType(BottomSheet),
       matching: find.byIcon(ZulipIcons.edit, skipOffstage: false));
@@ -1875,7 +1878,7 @@ void main() {
       await startEditInteractionFromActionSheet(tester, messageId: messageToEdit.id,
         originalRawContent: 'message to edit',
         delay: Duration.zero);
-      await tester.pump(const Duration(milliseconds: 250)); // bottom-sheet animation
+      await transitionDurationObserver.pumpPastTransition(tester); // bottom-sheet animation
 
       await tester.tap(failedMessageFinder);
       await tester.pump();
@@ -1899,7 +1902,7 @@ void main() {
       await startEditInteractionFromActionSheet(tester, messageId: messageToEdit.id,
         originalRawContent: 'message to edit',
         delay: Duration.zero);
-      await tester.pump(const Duration(milliseconds: 250)); // bottom-sheet animation
+      await transitionDurationObserver.pumpPastTransition(tester); // bottom-sheet animation
 
       await tester.tap(failedMessageFinder);
       await tester.pump();