After a slash command is deferred (interaction.response.defer), the
"thinking..." indicator persisted indefinitely because the code used
followup.send() which creates a separate message instead of replacing
or removing the deferred response.
Fix: use edit_original_response() to replace "thinking..." with the
confirmation text when provided, or delete_original_response() to
remove it when there is no confirmation. Also consolidated /reasoning
and /voice handlers to use _run_simple_slash instead of duplicating
the defer+dispatch pattern.
Fixes#3595.
0a80dd9c7a