fix: explicitly acknowledge UMB messages in receiver

Messages received from ActiveMQ were not being marked as read because
the receiver was closing the connection before auto-accept could
transmit the acknowledgment to the broker. Add explicit accept() call
after parsing each message to ensure proper acknowledgment.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: David Lanouette <dlanouet@redhat.com>

rh-pre-commit.version: 2.3.2
rh-pre-commit.check-secrets: ENABLED

Author: David Lanouette

src/pubtools/sign/clients/msg_recv_client.py

@@ -86,6 +86,7 @@ def on_message(self, event: proton.Event) -> None:
         outer_message = json.loads(event.message.body)
         headers = event.message.properties
         msg_id = outer_message["msg"][self.id_key]
+        self.accept(event.delivery)
 
         if msg_id in self.recv_ids:
             self.recv_ids[msg_id] = True