[http-client-csharp] We should not add "CollectionReturnResult" types into `typesToKeep` list

[ArcturusZhang]

We are adding collection return result types into the typesToKeep list:

typespec/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/ScmOutputLibrary.cs

Line 50 in de6f8eb

ScmCodeModelGenerator.Instance.AddTypeToKeep(scmMethod.CollectionDefinition);

Only "root types" should be able to add into the typesToKeep list.
The collection return result types are not really clients therefore they should not be added in the first place.
If a related operation is generated, they will be kept but if a related operation is not generated because of various reasons, those types should not be there.