Function decode_fill_parameters 

fn decode_fill_parameters(bytes: &[u8]) -> FieldValue

Decode a draft-20 FILL_PARAMETERS value: a nested parameter block in its own scope, reported the same way an outer block is.

The nesting is what makes this recursive with params_to_json — a fill’s LOCATION_FILTER is rendered exactly as the subscription’s is — and it bottoms out because Table 6 does not list FILL_PARAMETERS inside itself.