proof: ensure that recv ACK stream is closed correctly

This commit modifies the `HashMailBox.RecvAck` method to ensure that the
receive ACK gRPC stream is closed successfully.

proof/courier.go

@@ -439,6 +439,17 @@ func (h *HashMailBox) RecvAck(ctx context.Context, sid streamID) error {
 		return fmt.Errorf("unable to create read stream: %w", err)
 	}
 
+	// Ensure the read stream is closed before returning.
+	defer func() {
+		log.Debugf("Closing hashmail read stream (sid=%x)", sid[:])
+
+		err := readStream.CloseSend()
+		if err != nil {
+			log.Errorf("Unable to close hashmail read stream "+
+				"(sid=%x): %v", sid[:], err)
+		}
+	}()
+
 	log.Debugf("Exec stream Recv for receiver ACK (sid=%x)", sid[:])
 	msg, err := readStream.Recv()
 	if err != nil {