format the tree to current dart format spec

Mechanical `dart format` sweep across files that drifted from the
formatter's output (mostly trailing-comma and line-wrap differences
from a Dart SDK / formatter version bump). No semantic changes.

Caught because the pre-push gate now actually fires.

Co-Authored-By: Claude <noreply@anthropic.com>

test/widgets/multitab_pane_test.dart

@@ -20,8 +20,7 @@ MultitabEntry<String> entry(String id, {bool closeable = true, bool reorderable
   );
 }
 
-Widget body(BuildContext _, MultitabEntry<String> e) =>
-    SizedBox(key: ValueKey('body-${e.id}'), child: Text('body:${e.payload}'));
+Widget body(BuildContext _, MultitabEntry<String> e) => SizedBox(key: ValueKey('body-${e.id}'), child: Text('body:${e.payload}'));
 
 /// Stateful tap-counter body. Preserves a per-id count across rebuilds
 /// in a static map so the test can assert state survival across tab