Rework of #911 against dev with correctness fixes.
Public-facing channels (Facebook, Telegram, Discord, Feishu, WhatsApp,
Zalo OA, Zalo Personal, Pancake, Slack) no longer receive raw internal
error text when an agent run fails — an empty outbound is published
instead so channels still clean up placeholders / typing indicators.
Errors continue to be logged server-side at Error level.
Applied on both hot paths:
- cmd/gateway_consumer_normal.go (agent run failure)
- cmd/gateway_subagent_announce_queue.go (announce lead run failure)
Fixes vs #911:
- Whitelist uses channels.Type* constants (compile-time safe) instead
of string literals. The original PR's literals ("zalo", "line") did
not match any real channel type, so Zalo OA/Personal and Pancake
leaked errors while "line" was a dead branch.
- Added TypePancake and TypeSlack which were missing.
- Empty Content in Facebook.Send and Pancake.Send now short-circuits
(matching Telegram/Discord/Slack). Previously, suppressed errors
flowed to Graph/Pancake APIs as empty payloads, triggering 400s and
channel health degradation.
- Added table-driven test for isExternalChannel covering all real
channel types + empty + unknown + legacy "zalo" short form.
Closes#911
