_pop_slash_context fell back to a channel-only scan when the
_slash_user_id ContextVar was unset (i.e. send() invoked from a
non-slash code path such as a cron delivery or a normal channel reply).
That scan could steal another user's pending slash reply context: the
normal message got swallowed into an ephemeral response_url POST that
replaces the invoker's ack, and the slash invoker's actual reply then
posted publicly. Remove the fallback — when the ContextVar is unset,
match nothing.
Surgical reapply of PR #26788 (originally against gateway/platforms/slack.py).
bd707103ef