`#[derive(WpDerivedRequest)]`'s `contextual_get` should return error for non `SparseFoo` types

`contextual_get` works the same way as `#[derive(WpContextual)]` does, as it replaces `SparseFoo` with `FooWithEditContext`, `FooWithEmbedContext` & `FooWithViewContext`. To ensure that we always work with correct output types, we should return a compiler error if the `output` type of `WpDerivedRequest` doesn't have the `Sparse` prefix if the `contextual_get` attribute is used.