Follow-up to liuhao1024's #46924. Route plain-text approval replies
through the canonical /approve and /deny handlers (resolve thread, resume
typing, return localized confirmation) and deliver that confirmation back
to the user — previously a plain 'yes' resolved silently. Synthesize a
literal '/'-prefixed command so get_command_args() parses always/session
modifiers on every platform (is_command() only recognizes '/'). Add E2E
tests covering approve/deny/always/session vocab plus the no-pending and
unrelated-text fall-through cases.
