fix: 将断网场景的错误日志降级为 debug 级别

- Error closing connection 降级为 debug（TLS 关闭错误是断网预期行为）
- Write failed: batchBuffer is nil 降级为 debug
- Write operation failed 降级为 debug
- 避免长时间断网恢复时日志泛滥

Sources/Nats/NatsConnection.swift

@@ -987,7 +987,8 @@ class ConnectionHandler: ChannelInboundHandler {
                     try result.get()
                     self.fire(.disconnected)
                 } catch {
-                    logger.error("Error closing connection: \(error)")
+                    // 降级为 debug：网络断开时 TLS 无法完成正常关闭握手是预期行为
+                    logger.debug("Connection closed with error (will reconnect): \(error)")
                 }
                 // Only start reconnect after disconnect is complete
                 self.handleReconnect()
@@ -1060,7 +1061,8 @@ class ConnectionHandler: ChannelInboundHandler {
             // Trigger reconnect to recover
             let currentState = state.withLockedValue { $0 }
             if currentState == .connected {
-                logger.error("Write failed: batchBuffer is nil but state is connected, triggering reconnect")
+                // 降级为 debug：这是断网恢复场景的正常状态
+                logger.debug("Write failed: batchBuffer is nil, triggering reconnect")
                 handleDisconnect()
             }
             throw NatsError.ClientError.invalidConnection("not connected")
@@ -1071,7 +1073,8 @@ class ConnectionHandler: ChannelInboundHandler {
             // Trigger reconnect on write failure - connection may be broken
             let currentState = state.withLockedValue { $0 }
             if currentState == .connected {
-                logger.error("Write operation failed, triggering reconnect: \(error)")
+                // 降级为 debug：网络断开时写入失败是预期行为
+                logger.debug("Write operation failed, triggering reconnect: \(error)")
                 handleDisconnect()
             }
             throw NatsError.ClientError.io(error)