Fix test prototypes fields being marked as unused (#20853)

Content.IntegrationTests/TestPrototypesAttribute.cs

@@ -1,9 +1,12 @@
+using JetBrains.Annotations;
+
 namespace Content.IntegrationTests;
 
 /// <summary>
 /// Attribute that indicates that a string contains yaml prototype data that should be loaded by integration tests.
 /// </summary>
 [AttributeUsage(AttributeTargets.Field)]
+[MeansImplicitUse]
 public sealed class TestPrototypesAttribute : Attribute
 {
 }