[http-client-csharp] malformed code is generated on some kind of customization code

[ArcturusZhang]

Sometimes, in customized code, roslyn cannot analyze the correct namespace of some symbols (or maybe not?), and code like this is generated:

We need to fix this.
The above code was generated when we have the following customization:

To reproduce:
Prepare typespec:

model Foo {
	bar?: Bar;
}

model Bar {
	b?: string;
}

In the generated code, add the following:

[CodeGenSerialization(nameof(Bars), "bars"]
public partial class Foo
{
	public IList<Bar> Bars {get;}
}

then generate the code.

[JoshLove-msft]

@ArcturusZhang can you please add repro instructions?

[ArcturusZhang]

@ArcturusZhang can you please add repro instructions?

added some reproduction steps.