fix: Reset _completeCompleter in ticker (#2636)

I was running this code:
```dart
void attack() async {
  print('Attack started');
  current = PlayerState.attack;

  final attackTicker = animationTicker!;
  await attackTicker.completed;
  print('Attack ticker completed');

  attackTicker.reset();
  current = PlayerState.idle;
}
```
and `attackTicker.completed` would get stuck as permanently completed
after the first completion.

packages/flame/lib/src/sprite_animation_ticker.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
 import 'package:flame/components.dart';
+import 'package:meta/meta.dart';
 
 /// A helper class to make the [spriteAnimation] tick.
 class SpriteAnimationTicker {
@@ -30,7 +31,8 @@ class SpriteAnimationTicker {
   /// Registered method to be triggered when the animation complete.
   void Function()? onComplete;
 
-  Completer<void>? _completeCompleter;
+  @visibleForTesting
+  Completer<void>? completeCompleter;
 
   /// The current frame that should be displayed.
   SpriteAnimationFrame get currentFrame => spriteAnimation.frames[currentIndex];
@@ -54,9 +56,9 @@ class SpriteAnimationTicker {
       return Future.value();
     }
 
-    _completeCompleter ??= Completer<void>();
+    completeCompleter ??= Completer<void>();
 
-    return _completeCompleter!.future;
+    return completeCompleter!.future;
   }
 
   /// Resets the animation, like it would just have been created.
@@ -66,6 +68,11 @@ class SpriteAnimationTicker {
     currentIndex = 0;
     _done = false;
     _started = false;
+
+    // Reset completeCompleter if it's already completed
+    if (completeCompleter?.isCompleted ?? false) {
+      completeCompleter = null;
+    }
   }
 
   /// Sets this animation to be on the last frame.
@@ -127,7 +134,7 @@ class SpriteAnimationTicker {
         } else {
           _done = true;
           onComplete?.call();
-          _completeCompleter?.complete();
+          completeCompleter?.complete();
           return;
         }
       } else {

packages/flame/test/sprite_animation_ticker_test.dart

@@ -145,5 +145,22 @@ void main() {
         expectLater(animationTicker.completed, doesNotComplete);
       },
     );
+
+    test("completed doesn't complete after the animation is reset", () async {
+      final sprite = MockSprite();
+      final animationTicker = SpriteAnimation.spriteList(
+        [sprite],
+        stepTime: 1,
+        loop: false,
+      ).createTicker();
+
+      animationTicker.completed;
+      animationTicker.update(1);
+      expect(animationTicker.completeCompleter!.isCompleted, true);
+
+      animationTicker.reset();
+      animationTicker.completed;
+      expect(animationTicker.completeCompleter!.isCompleted, false);
+    });
   });
 }