Parse thread_id from explicit deliver target (e.g. telegram:-1003724596514:17)
and forward it to _send_to_platform and mirror_to_session.
Previously _resolve_delivery_target() always set thread_id=None when
parsing the platform:chat_id format, breaking cron job delivery to
specific Telegram topics.
Added tests:
- test_explicit_telegram_topic_target_with_thread_id
- test_explicit_telegram_chat_id_without_thread_id
Also updated CRONJOB_SCHEMA deliver description to document the
platform:chat_id:thread_id format.
Co-authored-by: Alex Ferrari <alex@thealexferrari.com>