fix(xiaomi): 修复 HyperOS 推送参数错误（channel_id → extra.channel_id）

旧 API 用 channel_id 字段，HyperOS V3 API 要求改用 extra.channel_id。
错误参数名导致所有小米推送返回 27001（channel相关信息不匹配）。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

push-service/src/main/java/com/xuqm/push/service/provider/XiaomiPushProvider.java

@@ -75,7 +75,8 @@ public class XiaomiPushProvider implements PushProvider {
             Integer notifyType = options != null ? options.notifyType() : null;
             form.append("&notify_type=").append(notifyType == null ? 1 : Math.max(notifyType, 0));
             if (!channelId.isBlank()) {
-                form.append("&channel_id=").append(URLEncoder.encode(channelId, StandardCharsets.UTF_8));
+                // HyperOS V3 API requires extra.channel_id (not channel_id)
+                form.append("&extra.channel_id=").append(URLEncoder.encode(channelId, StandardCharsets.UTF_8));
             }
             log.info("Xiaomi push request: pkg={} channelId={} notifyType={} tokenPrefix={}",
                     packageName, channelId.isBlank() ? "(none)" : channelId,