Reduce CPU time when initiating RepositoryCacheKeys (#18267)

* Avoid an unneeded lookups in the Keys dictionary when initiating key cache

* Add further comments and unit tests around updated code.

---------

Co-authored-by: Andy Butland <abutland73@gmail.com>

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Persistence/Repositories/RepositoryCacheKeysTests.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Umbraco.
+// See LICENSE for more details.
+
+using NUnit.Framework;
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Persistence.Repositories;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Core.PropertyEditors;
+
+[TestFixture]
+public class RepositoryCacheKeysTests
+{
+    [Test]
+    public void GetKey_Returns_Expected_Key_For_Type()
+    {
+        var key = RepositoryCacheKeys.GetKey<IContent>();
+        Assert.AreEqual("uRepo_IContent_", key);
+    }
+
+    [Test]
+    public void GetKey_Returns_Expected_Key_For_Type_And_Id()
+    {
+        var key = RepositoryCacheKeys.GetKey<IContent, int>(1000);
+        Assert.AreEqual("uRepo_IContent_1000", key);
+    }
+}