1.58.1
(fix): Preserve required-nullable fields that arrive as null when a decoded model is
marshaled again. Generated UnmarshalJSON now records which required-nullable
keys were present in the incoming JSON (via a per-type wire-name-to-field-bit
table) so that MarshalJSON re-emits them as null instead of dropping them.
Fields absent from the input stay absent, and freshly constructed values
(e.g. request bodies) serialize exactly as before.