fix(cron): mirror delivered job replies into chat sessions

[huangrichao2020]

Summary

Mirror successfully delivered cron job replies back into the target chat session.

On a long-running gateway, a scheduled job can deliver a message into Telegram/Matrix/Slack/etc., but the next normal message in that same chat may not have that cron reply in its session transcript. In practice that makes the agent look like it forgot the cronjob it just sent.

This keeps the fix small: after a delivery succeeds, reuse gateway.mirror.mirror_to_session() with source_label="cron". Failed deliveries do not write fake history. Media-only deliveries get a compact transcript placeholder so the chat at least remembers that the cronjob sent an attachment.

There is a larger draft in the same area (#10650). This PR is intentionally narrower and ready to review: it only covers the production footgun where delivered cron replies are invisible to follow-up chat context.

Validation

• uv run --extra dev pytest tests/cron/test_scheduler.py::TestDeliverResultWrapping::test_successful_delivery_mirrors_to_target_session tests/cron/test_scheduler.py::TestDeliverResultWrapping::test_failed_delivery_does_not_mirror_to_target_session tests/cron/test_scheduler.py::TestDeliverResultWrapping::test_origin_delivery_preserves_thread_id -q
• uv run --extra dev python -m py_compile cron/scheduler.py
• git diff --check