[http-client-csharp] extension method should be invoked as extension methods

[ArcturusZhang]

Here:

typespec/packages/http-client-csharp/generator/Microsoft.Generator.CSharp.ClientModel/src/Providers/ScmMethodProviderCollection.cs

Line 435 in 8774715

Return(ClientModelPlugin.Instance.TypeFactory.ClientResponseApi.ToExpression().FromResponse(client.PipelineProperty.Invoke(processMessageName, [message, requestOptionsParameter], isAsync, true))),

This ProcessMessage and ProcessMessageAsync methods are actually extension methods, but we are invoking them as if this was an instance method.
It works fine before the namespace change because our clients will always be in the same namespace as the helpers.
But after we have different namespaces, we have troubles on them.