fix visitor related issue in MethodProvider.

Author: haiyuazhang

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/MethodProvider.cs

@@ -100,6 +100,7 @@ public void Update(
             }
 
             Signature = updated.Signature;
+            XmlDocs = MethodProviderHelpers.BuildXmlDocs(Signature);
 
             if (BodyExpression != null)
             {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/OutputLibraryVisitorTests.cs

@@ -198,6 +198,7 @@ public void VisitMethodToRenameParameterName()
             testMethod.Accept(new MethodVisitor());
 
             Assert.AreEqual("newName", testMethod.Signature.Parameters.First().Name);
+            Assert.AreEqual("/// <param name=\"newName\"></param>\n", testMethod.XmlDocs.Parameters[0].ToDisplayString());
             Assert.AreEqual("return newName;\n", testMethod?.BodyStatements!.ToDisplayString());
         }