test: Use TransitionDurationObserver in compose_box_test

Adds TransitionDurationObserver initialization inside the
prepareComposeBox setup. Every test that calls prepareComposeBox now
automatically attaches an observer to the widget tree.

Replaces hardcoded transition delays with dynamic duration handling via
transitionDurationObserver.pumpPastTransition(tester).

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();
@@ -1718,8 +1721,8 @@ void main() {
   }) async {
     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));
+    // sheet appears onscreen
+    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();