Extract duplicate `MockCollect` implementations into a reusable
`MockProgramCollect` type. Conditionally require `Serialize` on the
`Groupped` trait when the `general_renderer` feature is enabled.

Simplify `strip_all_flags` by using `Vec::retain` instead of
reallocating. Update doc examples to use `ShellContext` methods directly
instead of the now-removed `ShellContextHelper`. Rename `as_picker` to
`to_picker` for consistency with Rust conventions. Mark doc tests as
`ignore` and add necessary imports.