The stale-session fallback in `_send_text_chunk` re-sends without
`context_token` via `continue`, which advanced the `for attempt in
range(retries + 1)` loop. With `send_chunk_retries=0` (the setting the
reporter used to surface the raw iLink error) the re-send never happened:
the loop ended with `last_error is None` and `send()` failed with an empty
AssertionError. Count only real failures in `attempt` so the tokenless
re-send is free; backoff timings and log counters are unchanged.
Also documents the `ret=-2 prepare failed` / `unknown error` stale-session
behaviour in the Weixin troubleshooting table.
Part of #112709
2995063bcd