Add Discord thread support to cron delivery and send_message_tool.
- _parse_target_ref: handle discord platform with chat_id:thread_id format
- _send_discord: add thread_id param, route to /channels/{thread_id}/messages
- _send_to_platform: pass thread_id through for Discord
- Discord adapter send(): read thread_id from metadata for gateway path
- Update tool schema description to document Discord thread targets
Cherry-picked from PR #7046 by pandacooming (maxyangcn).
Follow-up fixes:
- Restore proxy support (resolve_proxy_url/proxy_kwargs_for_aiohttp) that was
accidentally deleted — would have caused NameError at runtime
- Remove duplicate _DISCORD_TARGET_RE regex; reuse existing _TELEGRAM_TOPIC_TARGET_RE
via _NUMERIC_TOPIC_RE alias (identical pattern)
- Fix misleading test comments about Discord negative snowflake IDs
(Discord uses positive snowflakes; negative IDs are a Telegram convention)
- Rewrite misleading scheduler test that claimed to exercise home channel
fallback but actually tested the explicit platform:chat_id parsing path
19292eb8bf