Responses-style providers serialize providerPayload history items instead of the
visible message blocks when replaying native history. Include providerPayload in
the append-only per-message digest so payload-only history rewrites stop the
stable-prefix walk and re-sync the changed message before any later divergent
tail.
Add a regression where an assistant message keeps identical visible content and
id but changes its openaiResponsesHistory providerPayload while a later message
also diverges; syncMessages must preserve the prefix before the assistant and
refresh the assistant payload.
Fixes#3406
roboomp
2b68785e82