The webchat channel sent NO_REPLY as visible text to clients instead
of suppressing it. Other channels (Telegram, Discord) already filter
this token via the reply dispatcher, but the webchat streaming path
bypassed this check.
Fixes#16269
- Add isHeartbeat to AgentRunContext to track heartbeat runs
- Pass isHeartbeat flag through agent runner execution
- Suppress webchat broadcast (deltas + final) for heartbeat runs when showOk is false
- Webchat uses channels.defaults.heartbeat settings (no per-channel config)
- Default behavior: hide HEARTBEAT_OK from webchat (matches other channels)
This allows users to control whether heartbeat responses appear in
the webchat UI via channels.defaults.heartbeat.showOk (defaults to false).
