fix(push): 修复小米推送 restricted_package_name 硬编码问题

从推送配置中读取 Android 包名，配置缺失时记录警告并跳过推送，
不再使用 com.example.app 占位值导致推送无法送达。

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

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

@@ -50,11 +50,16 @@ public class XiaomiPushProvider implements PushProvider {
             log.warn("Xiaomi push not configured");
             return false;
         }
+        String packageName = resolveConfig(appId, "packageName", "");
+        if (packageName.isBlank()) {
+            log.warn("Xiaomi push skipped: packageName not configured for appId={}", appId);
+            return false;
+        }
         try {
             String form = "registration_id=" + URLEncoder.encode(token, StandardCharsets.UTF_8)
                     + "&title=" + URLEncoder.encode(title, StandardCharsets.UTF_8)
                     + "&description=" + URLEncoder.encode(body, StandardCharsets.UTF_8)
-                    + "&restricted_package_name=com.example.app"
+                    + "&restricted_package_name=" + URLEncoder.encode(packageName, StandardCharsets.UTF_8)
                     + "&notify_type=1";
             if (options != null && options.channelId() != null && !options.channelId().isBlank()) {
                 form += "&channel_id=" + URLEncoder.encode(options.channelId(), StandardCharsets.UTF_8);