Unable to customize base model of generated model if spec has a base model

[live1206]

In MTG, we can only customize base model if input model doesn't have a base model already.

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

Lines 245 to 250 in fecf4f2

	if (_inputModel.BaseModel == null)
	{
	// consider models that have been customized to inherit from a different model
	if (CustomCodeView?.BaseType != null)
	{
	var baseType = CustomCodeView.BaseType;

There are cases, we need to customize the base model even the spec has specified the base model.