fix: 修复 CMPP 协议字段容量与版本兼容性

gateway/internal/upstream/connection.go

@@ -16,6 +16,13 @@ import (
 // The reader and heartbeat goroutines share one connection lifecycle. Closing
 // the connection must wake pending submitters before scheduling pool recovery.
 
+// Caller holds c.mu across sequence selection, wire write and pending registration.
+func (c *connection) sequenceAvailable(sequence uint32) bool {
+	_, submit := c.pending[sequence]
+	_, heartbeat := c.heartbeatPending[sequence]
+	return !submit && !heartbeat
+}
+
 type connection struct {
 	channelID            string
 	config               queue.UpstreamConfig