Cherry-picked from PR #2017 by @simpolism. Fixes#2011.
Discord slash commands in threads were missing thread_id in the
SessionSource, causing them to route to the parent channel session.
Commands like /usage and /reset returned wrong data or affected the
wrong session.
Detects discord.Thread channels in _build_slash_event and sets
chat_type='thread' with thread_id. Two tests added.