The delivery target parser uses split(':', 1) which only splits on the
first colon. For the documented format platform:chat_id:thread_id
(e.g. 'telegram:-1001234567890:17585'), thread_id gets munged into
chat_id and is never extracted.
Fix: split(':', 2) to correctly extract all three parts. Also fix
to_string() to include thread_id for proper round-tripping.
The downstream plumbing in _deliver_to_platform() already handles
thread_id correctly (line 292-293) — it just never received a value.
49d7210fed