fix(sdk-update): 修复 checkAppUpdate NPE — 服务端 versionName 为 null 时崩溃

Gson 反序列化时会将 JSON null 注入 Kotlin non-null String 字段，绕过默认值。
引入内部 DTO UpdateInfoDto（所有字符串字段可空），转换为 UpdateInfo 时用 orEmpty() 补默认值。

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

sdk-update/src/main/java/com/xuqm/sdk/update/UpdateSDK.kt

@@ -184,7 +184,7 @@ object UpdateSDK {
         }
 
         runCatching {
-            api.checkUpdate(XuqmSDK.appKey, "ANDROID", versionCode, userId).data?.let { info ->
+            api.checkUpdate(XuqmSDK.appKey, "ANDROID", versionCode, userId).data?.toUpdateInfo()?.let { info ->
                 val normalized = info.copy(downloadUrl = normalizeDownloadUrl(info.downloadUrl) ?: info.downloadUrl)
                 val afterIgnore = if (!bypassIgnore && normalized.needsUpdate && !normalized.forceUpdate
                     && isVersionIgnored(context, normalized.versionCode)

sdk-update/src/main/java/com/xuqm/sdk/update/api/UpdateApi.kt

@@ -6,12 +6,39 @@ import retrofit2.http.Query
 
 data class ApiResponse<T>(val code: Int, val data: T?, val message: String)
 
-interface UpdateApi {
+/** 服务端响应的原始 DTO，所有字符串字段可为 null，防止 Gson 绕过 Kotlin 默认值引发 NPE。 */
+internal data class UpdateInfoDto(
+    val needsUpdate: Boolean = false,
+    val versionName: String? = null,
+    val versionCode: Int = 0,
+    val downloadUrl: String? = null,
+    val changeLog: String? = null,
+    val forceUpdate: Boolean = false,
+    val appStoreUrl: String? = null,
+    val marketUrl: String? = null,
+    val requiresLogin: Boolean = false,
+    val apkHash: String? = null,
+) {
+    fun toUpdateInfo() = UpdateInfo(
+        needsUpdate = needsUpdate,
+        versionName = versionName.orEmpty(),
+        versionCode = versionCode,
+        downloadUrl = downloadUrl.orEmpty(),
+        changeLog = changeLog.orEmpty(),
+        forceUpdate = forceUpdate,
+        appStoreUrl = appStoreUrl.orEmpty(),
+        marketUrl = marketUrl.orEmpty(),
+        requiresLogin = requiresLogin,
+        apkHash = apkHash,
+    )
+}
+
+internal interface UpdateApi {
     @GET("api/v1/updates/app/check")
     suspend fun checkUpdate(
         @Query("appKey") appKey: String,
         @Query("platform") platform: String,
         @Query("currentVersionCode") currentVersionCode: Int,
         @Query("userId") userId: String? = null,
-    ): ApiResponse<UpdateInfo>
+    ): ApiResponse<UpdateInfoDto>
 }