Validate ProtoIds in tests (#38745)

* Convert string literals to protoids in Content.Tests

* Convert string literals to protoids or consts in Content.IntegrationTests

* Fix linter failures
Tricksy static using misled me

Content.IntegrationTests/Tests/PostMapInitTest.cs

@@ -77,6 +77,8 @@ namespace Content.IntegrationTests.Tests
             "Exo",
         };
 
+        private static readonly ProtoId<EntityCategoryPrototype> DoNotMapCategory = "DoNotMap";
+
         /// <summary>
         /// Asserts that specific files have been saved as grids and not maps.
         /// </summary>
@@ -254,7 +256,7 @@ namespace Content.IntegrationTests.Tests
                 return;
 
             var yamlEntities = node["entities"];
-            if (!protoManager.TryIndex<EntityCategoryPrototype>("DoNotMap", out var dnmCategory))
+            if (!protoManager.TryIndex(DoNotMapCategory, out var dnmCategory))
                 return;
 
             Assert.Multiple(() =>