Fix default value of flattend property parameter for ModelFactory (#51799)

* reproduce issue 51754.

* Fix default value of flattend property parameter for ModelFactory

* regen

---------

Co-authored-by: haiyuan_zhang <[email protected]>

Author: live1206

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

@@ -208,20 +208,26 @@ private void UpdateModelFactoryMethod(MethodProvider method, CSharpType returnTy
         {
             var updatedParameters = new List<ParameterProvider>(method.Signature.Parameters.Count);
             var updated = false;
+            var parameterShouldBeNullable = false; // if the previous method parameter is nullable, we need to ensure that the current parameter is also set with default value
             foreach (var parameter in method.Signature.Parameters)
             {
                 if (propertyMap.TryGetValue(parameter.Type, out var flattenedProperty))
                 {
                     updated = true;
                     var updatedParameter = flattenedProperty.AsParameter;
-                    if (flattenedProperty.Type.IsNullable)
+                    if (parameterShouldBeNullable || flattenedProperty.Type.IsNullable)
                     {
+                        parameterShouldBeNullable = true;
                         updatedParameter.DefaultValue = Default; // Ensure that the default value is set to null for nullable types
                     }
                     updatedParameters.Add(updatedParameter);
                 }
                 else
                 {
+                    if (parameter.Type.IsNullable)
+                    {
+                        parameterShouldBeNullable = true;
+                    }
                     updatedParameters.Add(parameter);
                 }
             }

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

@@ -71,8 +71,15 @@ interface FooSettingsOperations {
 }
 
 model FooSettingsProperties {
-  accessControlEnabled: boolean;
+  accessControlEnabled?: boolean;
 
   @visibility(Lifecycle.Read)
   provisioningState?: ResourceProvisioningState;
+
+  @visibility(Lifecycle.Read)
+  metaData?: FooSettingsPropertiesMetaData;
+}
+
+model FooSettingsPropertiesMetaData {
+  metaDatas?: string[];
 }