Prevented type descriptions from being overwritten while merging (#7751)

Author: glen-84

src/HotChocolate/Core/src/Types/Types/Descriptors/Definitions/DefinitionBase.cs

@@ -199,7 +199,7 @@ protected void MergeInto(DefinitionBase target)
             }
         }
 
-        if (Description is not null)
+        if (target.Description is null && Description is not null)
         {
             target.Description = Description;
         }

src/HotChocolate/Core/test/Types.Tests.Documentation/QueryExtWithDocs1.cs

@@ -0,0 +1,10 @@
+namespace HotChocolate.Types.Descriptors;
+
+/// <summary>
+/// QueryExtWithDocs1
+/// </summary>
+[QueryType]
+public static class QueryExtWithDocs1
+{
+    public static int GetFoo1() => 1;
+}

src/HotChocolate/Core/test/Types.Tests.Documentation/QueryExtWithDocs2.cs

@@ -0,0 +1,10 @@
+namespace HotChocolate.Types.Descriptors;
+
+/// <summary>
+/// QueryExtWithDocs2
+/// </summary>
+[QueryType]
+public static class QueryExtWithDocs2
+{
+    public static int GetFoo2() => 1;
+}

src/HotChocolate/Core/test/Types.Tests/Types/Descriptors/Definitions/DefinitionBaseTests.cs

@@ -0,0 +1,34 @@
+using CookieCrumble;
+using HotChocolate.Execution;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace HotChocolate.Types.Descriptors.Definitions;
+
+public class DefinitionBaseTests
+{
+    [Fact]
+    public async Task MergeInto_MultipleTypeExtensionXmlDescriptions_UsesQueryTypeDescription()
+    {
+        // arrange & act
+        var schema = await new ServiceCollection()
+            .AddGraphQL()
+            .AddQueryType(d => d.Description("Query"))
+            .AddTypeExtension(typeof(QueryExtWithDocs1))
+            .AddTypeExtension(typeof(QueryExtWithDocs2))
+            .BuildSchemaAsync();
+
+        // assert
+        schema.MatchInlineSnapshot(
+            """
+            schema {
+              query: Query
+            }
+
+            "Query"
+            type Query {
+              foo1: Int!
+              foo2: Int!
+            }
+            """);
+    }
+}