Update MockFragment Stats method

The changes include synchronizing access to the underlying map when calculating the Length stat in the MockFragment's Stats() method. This is to ensure thread safety while accessing shared resources.

internal/testutil/mockfragment/mockfragment.go

@@ -44,7 +44,11 @@ func New() *MockFragment {
 }
 
 func (f *MockFragment) Stats() storage.Stats {
-	return storage.Stats{}
+	f.Lock()
+	defer f.Unlock()
+	return storage.Stats{
+		Length: len(f.m),
+	}
 }
 
 func (f *MockFragment) Name() string {