sanitizeRehydratedOpenAIResponsesAssistantMessage now removes the
assistant providerPayload for Responses-family messages, not just the
stale thinkingSignature. After rehydration the native replay snapshot
belongs to a previous live provider connection and replaying it on a
warmed session causes 401 rejections from GitHub Copilot.
User/developer providerPayload is preserved by the caller since it
carries durable compaction and preserved history needed for valid cold
request reconstruction.
Fixes#592
Vu Anh Nguyen
3860ac986c