Preserve existing Examine FieldDefinitionCollection if it already exists (#20931)

* Preserve existing Examine FieldDefinitionCollection if it already exists (#20267)

* Fix missing bracket

* Minor tidy/addition of comments; addition of unit tests.

---------

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

tests/Umbraco.Tests.UnitTests/Umbraco.Infrastructure/Examine/UmbracoFieldDefinitionCollectionTests.cs

@@ -0,0 +1,75 @@
+using Examine;
+using NUnit.Framework;
+using Umbraco.Cms.Infrastructure.Examine;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Infrastructure.Examine;
+
+[TestFixture]
+internal class UmbracoFieldDefinitionCollectionTests
+{
+    [Test]
+    public void Create_Contains_Expected_Fields()
+    {
+        var collection = new UmbracoFieldDefinitionCollection();
+        AssertDefaultField(collection);
+    }
+
+    [Test]
+    public void Create_New_Contains_Expected_Fields()
+    {
+        var collection = new UmbracoFieldDefinitionCollection();
+        collection.AddOrUpdate(new FieldDefinition("customField", "string"));
+        var collectionCount = collection.Count;
+
+        collection = new UmbracoFieldDefinitionCollection();
+        Assert.AreEqual(collectionCount - 1, collection.Count);
+        AssertDefaultField(collection);
+        AssertCustomField(collection, expectExists: false);
+    }
+
+    [Test]
+    public void Create_With_Existing_Contains_Expected_Fields()
+    {
+        var collection = new UmbracoFieldDefinitionCollection();
+        collection.AddOrUpdate(new FieldDefinition("customField", "string"));
+        var collectionCount = collection.Count;
+
+        collection = new UmbracoFieldDefinitionCollection(collection);
+        Assert.AreEqual(collectionCount, collection.Count);
+        AssertDefaultField(collection);
+        AssertCustomField(collection, expectExists: true);
+    }
+
+    [Test]
+    public void Create_With_Existing_Retains_Override_Of_DefaultField()
+    {
+        var collection = new UmbracoFieldDefinitionCollection();
+        collection.AddOrUpdate(new FieldDefinition("parentID", "string"));
+
+        collection = new UmbracoFieldDefinitionCollection(collection);
+        AssertDefaultField(collection, "string");
+    }
+
+    private static void AssertDefaultField(UmbracoFieldDefinitionCollection collection, string expectedType = "int")
+    {
+        var field = collection.SingleOrDefault(x => x.Name == "parentID");
+        Assert.IsNotNull(field);
+        Assert.AreEqual("parentID", field.Name);
+        Assert.AreEqual(expectedType, field.Type);
+    }
+
+    private static void AssertCustomField(UmbracoFieldDefinitionCollection collection, bool expectExists)
+    {
+        var field = collection.SingleOrDefault(x => x.Name == "customField");
+        if (expectExists is false)
+        {
+            Assert.IsNull(field.Name);
+            Assert.IsNull(field.Type);
+            return;
+        }
+
+        Assert.IsNotNull(field);
+        Assert.AreEqual("customField", field.Name);
+        Assert.AreEqual("string", field.Type);
+    }
+}