Two Mattermost thread-related bugs:
1. _resolve_root_id() — Mattermost CRT requires root_id to be the
thread root post. Using any reply's own ID as root_id causes
'400 Invalid RootId'. Add _resolve_root_id() that walks up the
post chain via API to find the actual root, and apply it in
send(), _send_url_as_file(), and _send_local_file().
2. _progress_reply_to — The condition in run.py only checked
Platform.FEISHU, missing Mattermost entirely. This caused tool
progress messages to always land in the main channel instead of
the thread. Add Platform.MATTERMOST to the condition so
progress messages are routed to threads when reply_mode=thread.
Impact: Tool progress messages now appear in Mattermost threads
instead of flooding the main channel; thread replies no longer
fail with Invalid RootId when the reply target is itself a reply.