Say when the provider lost the conversation, and offer a retry

When Codex loses a session’s thread, the executor ends the turn and forgets the thread. The app did not know that failure and showed “Something went wrong talking to Codex.” with no way forward.

The callout is now titled “Conversation not found” and explains that the provider no longer has this conversation. Retry resends your message, and the executor starts it in a new thread. The session keeps its history and branch, which a new session would not.