Add early auth check in _handle_slack_message() that runs BEFORE any
API calls (thread context fetch, user name resolution, file downloads)
or file processing. Unauthorized users could previously trigger Slack
API calls and file downloads before the runner's _is_user_authorized
gate rejected them.
Same pattern as Telegram fix#54164: build a SessionSource and check
the runner's _is_user_authorized at the adapter level before event
construction consumes resources.
Fixes the gap where Slack has no adapter-level auth gate while Discord
and Telegram have adapter-level allowlists.
5bb933eedf