Fix to namespace visitor where types were moved out of generated dir (Azure#51999)

Author: JoshLove-msft

eng/packages/http-client-csharp/generator/Azure.Generator/src/Visitors/NamespaceVisitor.cs

@@ -73,7 +73,8 @@ private static void UpdateModelsNamespace(TypeProvider type)
                 // here to make diffs easier to review while migrating. Calculate the fileName as it won't always match the Name
                 // property, e.g. for serialization providers.
                 // https://github.com/Azure/azure-sdk-for-net/issues/50286
-                if (type.RelativeFilePath.Contains("Models"))
+                var separator = Path.DirectorySeparatorChar;
+                if (type.RelativeFilePath.Contains($"Generated{separator}Models{separator}"))
                 {
                     var fileName = Path.GetRelativePath(Path.Combine("src", "Generated", "Models"),
                         type.RelativeFilePath);

eng/packages/http-client-csharp/generator/Azure.Generator/test/Visitors/NamespaceVisitorTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System.IO;
 using Microsoft.TypeSpec.Generator.Input;
 using Microsoft.TypeSpec.Generator.Providers;
 using NUnit.Framework;
@@ -38,6 +39,38 @@ public void DoesNotUseModelsNamespaceIfConfigSetToFalse()
             Assert.AreEqual("Samples", updatedModel!.Type.Namespace);
         }
 
+        [Test]
+        public void MovesModelsOutOfModelsDirectory()
+        {
+            MockHelpers.LoadMockGenerator(configurationJson: "{ \"package-name\": \"TestLibrary\", \"model-namespace\": false }");
+            var visitor = new TestNamespaceVisitor();
+            var inputType = InputFactory.Model("TestModel", "Samples");
+            var model = new ModelProvider(inputType);
+            var updatedModel = visitor.InvokePreVisitModel(inputType, model);
+
+            Assert.IsNotNull(updatedModel);
+            Assert.AreEqual(
+                $"src{Path.DirectorySeparatorChar}Generated{Path.DirectorySeparatorChar}{updatedModel!.Name}.cs",
+                updatedModel.RelativeFilePath);
+        }
+
+        [Test]
+        public void DoesNotMoveModelIfNotInModelsDirectory()
+        {
+            MockHelpers.LoadMockGenerator(configurationJson: "{ \"package-name\": \"TestLibrary\", \"model-namespace\": false }");
+            var visitor = new TestNamespaceVisitor();
+            var inputType = InputFactory.Model("TestModels", "Samples");
+            var model = new ModelProvider(inputType);
+
+            model.Update(relativeFilePath: $"src{Path.DirectorySeparatorChar}Generated{Path.DirectorySeparatorChar}{model.Name}.cs");
+            var updatedModel = visitor.InvokePreVisitModel(inputType, model);
+
+            Assert.IsNotNull(updatedModel);
+            Assert.AreEqual(
+                $"src{Path.DirectorySeparatorChar}Generated{Path.DirectorySeparatorChar}{updatedModel!.Name}.cs",
+                updatedModel.RelativeFilePath);
+        }
+
         [Test]
         public void DoesNotChangeNamespaceOfCustomizedModel()
         {