Progress/status callbacks (context-pressure, compression retries,
model fallback) route through _send_or_update_status_coro, which
edits the previous bubble for the same status_key when the adapter
implements send_or_update_status — but only Telegram did. On Slack
every status event posted a fresh thread message, so a compression
retry loop spammed a dozen out-of-order bubbles into the thread
('Context too large 1/3... 2/3... 3/3', fallback switches, etc.).
Implement send_or_update_status on the Slack adapter following the
Telegram pattern (#30045): first call posts and caches the message ts
per (channel, thread, status_key); subsequent calls edit that message
via chat.update. Edit failure drops the cached ts and falls back to a
fresh send. Cache is FIFO-bounded.
