Recover a conversation after the TypeScript client loses its create response

A conversation creation POST can succeed on the server while the TypeScript client loses its response or times out during startup. The caller then sees a failure even though the initial message may already have run; repeating the POST can repeat work.

Acceptance criteria:

• When the caller supplied a conversation ID and the POST fails without an HTTP response, reconcile through bounded GET requests for that ID.
• Never replay the creation POST or its initial message.
• Preserve explicit HTTP failures and the original transport failure if reconciliation cannot succeed; stop on authorization errors.
• Cover dropped responses and delayed creation using a real HTTP server.

Extracted from #4966 so response recovery can be reviewed independently of conversation-scoped runtime routing. Ready for development.