fix(test): cancel one-shot Timers on CacheFiller / MutationReplayer dispose

Smoke test failed: "A Timer is still pending even after the widget
tree was disposed." Both workers fired their initial-delay Timer
via `Timer(duration, _sweep)` and stored only the periodic ticker
in the cancellable field — the one-shot Timer leaked past dispose
and tripped the test framework's invariant check.

Track both as _initialTimer + _intervalTimer; cancel both in
dispose(). Behavior is unchanged in production (ref.onDispose only
fires on process death normally); this is purely a test-harness
fix.

flutter_client/lib/cache/cache_filler.dart

@@ -37,7 +37,8 @@ class CacheFiller {
   CacheFiller(this._ref);
   final Ref _ref;
 
-  Timer? _timer;
+  Timer? _initialTimer;
+  Timer? _intervalTimer;
   bool _running = false;
   bool _disposed = false;
 
@@ -61,8 +62,8 @@ class CacheFiller {
   static const _maxIdsPerSweep = 200;
 
   void start() {
-    Timer(_initialDelay, _sweep);
-    _timer = Timer.periodic(_interval, (_) => _sweep());
+    _initialTimer = Timer(_initialDelay, _sweep);
+    _intervalTimer = Timer.periodic(_interval, (_) => _sweep());
   }
 
   Future<void> _sweep() async {
@@ -217,7 +218,8 @@ class CacheFiller {
 
   void dispose() {
     _disposed = true;
-    _timer?.cancel();
+    _initialTimer?.cancel();
+    _intervalTimer?.cancel();
   }
 }