When chat_with_retry returns an error response (finish_reason='error')
instead of raising an exception, archive() previously treated the error
message as a valid summary and wrote it to history.jsonl, while the
original session data was already cleared by /new — causing irreversible
data loss.
Fix: check finish_reason after the LLM call and raise RuntimeError on
error responses, which naturally falls through to the existing raw_archive
fallback. This preserves the original messages in history.jsonl instead
of losing them.
Fixes#3244