fix(cron): stream cron reminders with stream_id and turn_end

[chengyongru]

Fixes #3718

When cron jobs trigger agent responses, they bypass AgentLoop._dispatch which normally creates streaming callbacks (on_stream/on_stream_end) with _stream_id. As a result, WebSocket clients never receive stream_id in delta events and can't properly associate streaming fragments.

This change replicates the streaming logic in on_cron_job:

• Detects if the target channel supports streaming
• Generates _stream_id segments (base_id:segment)
• Emits _stream_delta, _stream_end, _streamed, and _turn_end metadata
• Prevents duplicate final delivery when streaming was used

Also adds a test verifying the full streaming lifecycle for cron-triggered messages.