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

[oguzkocer]

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.