fix(restart): deliver completion after channel reconnects (#4931)

docs/channel-plugin-guide.md

@@ -197,7 +197,19 @@ The agent receives the message and processes it. Replies arrive in your `send()`
 |--------|-------------|
 | `async start()` | **Must block forever.** Connect to platform, listen for messages, call `_handle_message()` on each. If this returns, the channel is dead. |
 | `async stop()` | Set `self._running = False` and clean up. Called when gateway shuts down. |
-| `async send(msg: OutboundMessage)` | Deliver an outbound message to the platform. |
+| `async send(msg: OutboundMessage)` | Deliver an outbound message to the platform. Raise when the transport does not accept it. |
+
+#### Outbound delivery contract
+
+A normal return from `send()` means either the visible payload was accepted by the
+platform transport/API, or the channel deliberately had nothing to deliver (for example,
+an empty progress event). Do not log and return when the client is disconnected, still
+starting, or the platform rejects the request. Raise an exception so `ChannelManager` can
+apply the shared retry policy.
+
+`send()` may run as soon as `is_running` becomes true. If a channel sets `_running` before
+its transport is ready, it must keep raising until delivery can be attempted safely. Small
+platform-specific retries are fine, but the final failure must still reach the manager.
 
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