Fix stackoverflow during property flatten (Azure#52899)

Author: live1206

eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Visitors/FlattenPropertyVisitor.cs

@@ -239,9 +239,16 @@ int GetAdditionalPropertyIndex()
         // This dictionary holds the flattened model types, where the key is the CSharpType of the model and the value is a dictionary of property names to flattened PropertyProvider.
         // So that, we can use this to update the model factory methods later.
         private readonly Dictionary<CSharpType, (Dictionary<string, List<FlattenPropertyInfo>>, Dictionary<CSharpType, List<FlattenPropertyInfo>>)> _flattenedModelTypes = new(new CSharpTypeNameComparer());
-
+        private readonly HashSet<CSharpType> _visitedModelTypes = new();
         private void FlattenModel(ModelProvider model)
         {
+            if (_visitedModelTypes.Contains(model.Type))
+            {
+                // already visiting this model type, we have a cycle, return
+                return;
+            }
+            _visitedModelTypes.Add(model.Type);
+
             var isFlattenProperty = false;
             var isSafeFlatten = false;
             var propertyMap = new Dictionary<PropertyProvider, List<FlattenPropertyInfo>>();

eng/packages/http-client-csharp-mgmt/generator/TestProjects/Local/Mgmt-TypeSpec/foo.tsp

@@ -33,6 +33,12 @@ model FooProperties {
 
   prop1: string[];
   prop2?: int32[];
+
+  nestedProperty: NestedFooModel;
+}
+
+model NestedFooModel {
+  properties: FooProperties;
 }
 
 @armResourceOperations