Each Slack thread now gets its own conversation session instead of
sharing one session per channel. DM sessions are unchanged.
Added as a generic feature to also support if Feishu threads support
is added in the future.
Use files_upload_v2 API to upload media attachments in Slack messages.
This enables the message tool's media parameter to work correctly
when sending images or other files through the Slack channel.
Requires files:write OAuth scope.
Paul