Update changelogs from fern repo (#6945)

Co-authored-by: thesandlord <8902396+thesandlord@users.noreply.github.com>

Author: thesandlord

fern/products/sdks/generators/go/changelog/2026-09-14.mdx

@@ -0,0 +1,9 @@
+## 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.
+
+