Silent/NO_REPLY responses leak debug fallback message to channels

[f-liva]

Bug

When an agent intentionally returns NO_REPLY (empty response with silent: true), the system generates a debug fallback message like:

[The agent completed processing but returned no text response. (3 in / 8 out | 1 iter)]

This message is sent back to the channel (WhatsApp, Telegram, etc.) instead of being suppressed.

Root Cause

• routes.rs: The REST endpoint checks for empty responses but doesn't check the silent field from AgentLoopResult, generating a debug fallback instead.
• ws.rs: The WebSocket handler has the same issue — empty responses get a fallback message even when the agent chose to be silent.
• types.rs: MessageResponse struct was missing the silent field.

Expected Behavior

When an agent returns NO_REPLY / silent response:

1. The API should return { "silent": true } in the response
2. Channel gateways should not send any message to the user

Affected Channels

• WhatsApp (gateway sends the debug text as a regular message)
• Any channel using the REST or WebSocket API