Reference wxclawbot-cli docs: ret=-2 is a rate limit (~7 msgs / 5 min
per bot), NOT a stale session signal. Empty/missing errmsg is the
normal rate-limit response; only 'unknown error' correlates with stale
session per hermes-agent.
Changes:
- _is_stale_session_ret: only match 'unknown error', not empty errmsg
- _send_text/_send_media_file: on ret=-2 wait 60s then retry once
instead of retrying without context_token
- Remove stale-session retry for empty errmsg (was burning quota)
- Update tests to cover rate-limit backoff path