Route both ProtobufSerializer push_back sites through one helper

`insertDefaults` had the same unguarded `push_back` as `readRow`, which the
previous commit fixed in only one of the two places. Both now go through
`pushBackOrDestroy` so the ownership ordering cannot drift apart again. Still
not reproduced, same as before: the window is one allocation wide.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>