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/Commands/RejuvenateTest.cs

@@ -15,6 +15,8 @@ namespace Content.IntegrationTests.Tests.Commands
     [TestOf(typeof(RejuvenateSystem))]
     public sealed class RejuvenateTest
     {
+        private static readonly ProtoId<DamageGroupPrototype> TestDamageGroup = "Toxin";
+
         [TestPrototypes]
         private const string Prototypes = @"
 - type: entity
@@ -62,7 +64,7 @@ namespace Content.IntegrationTests.Tests.Commands
                 });
 
                 // Kill the entity
-                DamageSpecifier damage = new(prototypeManager.Index<DamageGroupPrototype>("Toxin"), FixedPoint2.New(10000000));
+                DamageSpecifier damage = new(prototypeManager.Index(TestDamageGroup), FixedPoint2.New(10000000));
 
                 damSystem.TryChangeDamage(human, damage, true);