cursor: say what Future.sync defers, and give the default cursor time on a slow runner

The comment on the fetch in restorePixels read as if the fetch were put
off; it runs at once, and Future.sync only turns its throw into an error
the await hands over later, which is what keeps the notify out of the
build.

cursor_default_test gave the default cursor 100 ms to decode, which a
slow runner may miss. It cannot wait for the cursor itself, since asking
for it would make it; it now waits 500 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

flutter/lib/models/model.dart

@@ -3742,7 +3742,8 @@ class CursorModel with ChangeNotifier {
     final generation = _generation;
     () async {
       try {
-        // Never at once: a build may be what asked, and a failure tells the listeners.
+        // A throw becomes an error the await hands over later, not during the build that may
+        // have asked: a failure tells the listeners.
         final shape = await Future.sync(() => fetchCursorShape(id));
         if (generation != _generation) {
           return;

flutter/test/cursor_default_test.dart

@@ -54,7 +54,8 @@ void main() {
     final evt = {'id': '2'};
     ffi.ffiModel.updateLastCursorId(evt);
     ffi.ffiModel.handleCursorId(evt);
-    await Future<void>.delayed(const Duration(milliseconds: 100));
+    // Not a wait for the default cursor itself: asking for it would make it.
+    await Future<void>.delayed(const Duration(milliseconds: 500));
     expect(cursor.cache, isNull);
     final built = _key(buildCursorOfCache(
         cursor, 1.0, cursor.cache ?? preDefaultCursor.cache));