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## 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.