fix(backend): Handle stale RabbitMQ channels on connection drop

When the RabbitMQ connection drops, connect_robust reconnects the
underlying connection but the cached channel object becomes invalid.
The is_ready check passes (channel doesn't report as closed) but
operations fail with ChannelInvalidStateError ('No active transport
in channel'), generating ~39K Sentry errors (AUTOGPT-SERVER-1TN).

Fix:
- Add _ensure_channel() helper that resets stale channels before
  reconnecting, so connect() creates a fresh channel
- Catch ChannelInvalidStateError in publish_message and do a full
  reconnect (reset both channel and connection) before retrying
- Simplify get_channel() to use the same resilient helper

The existing @func_retry decorator provides additional retry
resilience on top of the explicit reconnect handling.

autogpt_platform/backend/backend/data/rabbitmq.py

@@ -291,6 +291,18 @@ class AsyncRabbitMQ(RabbitMQBase):
                     exchange, routing_key=queue.routing_key or queue.name
                 )
 
+    async def _ensure_channel(self) -> aio_pika.abc.AbstractChannel:
+        """Get a valid channel, reconnecting if the current one is stale."""
+        if not self.is_ready:
+            # Reset stale channel so connect() creates a fresh one
+            self._channel = None
+            await self.connect()
+
+        if self._channel is None:
+            raise RuntimeError("Channel should be established after connect")
+
+        return self._channel
+
     @func_retry
     async def publish_message(
         self,
@@ -299,32 +311,58 @@ class AsyncRabbitMQ(RabbitMQBase):
         exchange: Optional[Exchange] = None,
         persistent: bool = True,
     ) -> None:
-        if not self.is_ready:
+        try:
+            channel = await self._ensure_channel()
+        except Exception:
+            # Force full reconnect on channel acquisition failure
+            self._channel = None
+            self._connection = None
+            channel = await self._ensure_channel()
+
+        try:
+            if exchange:
+                exchange_obj = await channel.get_exchange(exchange.name)
+            else:
+                exchange_obj = channel.default_exchange
+
+            await exchange_obj.publish(
+                aio_pika.Message(
+                    body=message.encode(),
+                    delivery_mode=(
+                        aio_pika.DeliveryMode.PERSISTENT
+                        if persistent
+                        else aio_pika.DeliveryMode.NOT_PERSISTENT
+                    ),
+                ),
+                routing_key=routing_key,
+            )
+        except aio_pika.exceptions.ChannelInvalidStateError:
+            logger.warning(
+                "RabbitMQ channel invalid, reconnecting and retrying publish"
+            )
+            self._channel = None
+            self._connection = None
             await self.connect()
 
-        if self._channel is None:
-            raise RuntimeError("Channel should be established after connect")
+            if self._channel is None:
+                raise RuntimeError("Channel should be established after reconnect")
 
-        if exchange:
-            exchange_obj = await self._channel.get_exchange(exchange.name)
-        else:
-            exchange_obj = self._channel.default_exchange
+            if exchange:
+                exchange_obj = await self._channel.get_exchange(exchange.name)
+            else:
+                exchange_obj = self._channel.default_exchange
 
-        await exchange_obj.publish(
-            aio_pika.Message(
-                body=message.encode(),
-                delivery_mode=(
-                    aio_pika.DeliveryMode.PERSISTENT
-                    if persistent
-                    else aio_pika.DeliveryMode.NOT_PERSISTENT
+            await exchange_obj.publish(
+                aio_pika.Message(
+                    body=message.encode(),
+                    delivery_mode=(
+                        aio_pika.DeliveryMode.PERSISTENT
+                        if persistent
+                        else aio_pika.DeliveryMode.NOT_PERSISTENT
+                    ),
                 ),
-            ),
-            routing_key=routing_key,
-        )
+                routing_key=routing_key,
+            )
 
     async def get_channel(self) -> aio_pika.abc.AbstractChannel:
-        if not self.is_ready:
-            await self.connect()
-        if self._channel is None:
-            raise RuntimeError("Channel should be established after connect")
-        return self._channel
+        return await self._ensure_channel()