_resolve_thread_id() falls back to _last_inbound_thread[chat_id] when no
explicit thread is present. That fallback exists for interactive DMs, where
Google Chat spawns a fresh thread per top-level user message and the adapter
drops thread_id to keep the session key stable. It also fired for cron
deliveries, which carry job_id in their metadata but no thread: the output
landed as a reply inside the last inbound thread instead of starting a new
top-level message.
Bypass the _last_inbound_thread fallback when metadata has a job_id (i.e. the
message is an automated cron delivery), so cron output posts at top level
unless an explicit thread is requested.
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