Function strip_unmodelled_sampler_keys 

fn strip_unmodelled_sampler_keys(
    body: &mut Value,
    trust_client_sampling: bool,
) -> Vec<String>

Remove UNMODELLED_SAMPLER_KEYS from an untrusted body, returning what was removed so it joins the discard record.

A no-op when the client is trusted — trusted means trusted, unmodelled keys included — and on a body that is not a JSON object, which the rest of the pipeline also leaves alone.