fix(cron): support Telegram topic delivery via platform:chat_id:thread_id format (#2455)

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>

cron/scheduler.py

@@ -80,11 +80,16 @@ def _resolve_delivery_target(job: dict) -> Optional[dict]:
         }
 
     if ":" in deliver:
-        platform_name, chat_id = deliver.split(":", 1)
+        platform_name, rest = deliver.split(":", 1)
+        # Check for thread_id suffix (e.g. "telegram:-1003724596514:17")
+        if ":" in rest:
+            chat_id, thread_id = rest.split(":", 1)
+        else:
+            chat_id, thread_id = rest, None
         return {
             "platform": platform_name,
             "chat_id": chat_id,
-            "thread_id": None,
+            "thread_id": thread_id,
         }
 
     platform_name = deliver