refactor(ingress): 信箱连续上界遍历改用解构

contiguousUpTo 由 row.msgId 改为 for ((msgId, _) in rows)；MailboxRow 是 data class，
取值结果相同，行为不变，无配置与迁移影响。

验证：./gradlew test 通过。

src/main/kotlin/com/gzzn/omms/msgexchange/ingress/InboxPoller.kt

@@ -108,9 +108,9 @@ class InboxPoller(
     private fun contiguousUpTo(from: Long, rows: List<MailboxRow>): Long? {
         var expected = from + 1
         var last: Long? = null
-        for (row in rows) {
-            if (row.msgId != expected) break
-            last = row.msgId
+        for ((msgId, _) in rows) {
+            if (msgId != expected) break
+            last = msgId
             expected++
         }
         return last