feat: refactor super admin to use is_super_admin flag and enhance site deletion logic

- Changed super admin detection from role-based to `is_super_admin` flag in AdminUser model
- Added `requireDefaultAdminSiteId()` method to throw validation error when no integration site exists
- Enhanced site deletion to migrate platform role bindings to fallback site and auto-delete site-specific admin accounts
- Made agent line code optional with auto-generation fallback using `{site_code}-agent-{counter}` format

tests/Pest.php

@@ -50,30 +50,11 @@ expect()->extend('toBeOne', function () {
 |
 */
 
-/** 为后台测试账号挂上 `super_admin` 角色（细粒度权限校验全放行）。 */
+/** 为后台测试账号挂上唯一超级管理员（不绑定站点）。 */
 function grantSuperAdminRole(AdminUser $admin): void
 {
-    $now = now();
-    DB::table('admin_roles')->updateOrInsert(
-        ['slug' => AdminUser::ROLE_SUPER_ADMIN],
-        [
-            'name' => 'Super Admin',
-            'code' => AdminUser::ROLE_SUPER_ADMIN,
-            'created_at' => $now,
-            'updated_at' => $now,
-        ],
-    );
-    $rid = (int) DB::table('admin_roles')->where('slug', AdminUser::ROLE_SUPER_ADMIN)->value('id');
-    $siteId = (int) DB::table('admin_sites')->where('is_default', true)->value('id');
-
-    DB::table('admin_user_site_roles')->updateOrInsert(
-        [
-            'admin_user_id' => $admin->id,
-            'site_id' => $siteId,
-            'role_id' => $rid,
-        ],
-        ['granted_at' => $now],
-    );
+    \App\Support\PlatformSystemRoles::ensureSuperAdminRole();
+    \App\Support\SuperAdminAccount::assign($admin);
 }
 
 /** 为后台测试账号挂上代理节点（需已存在 agent_nodes / admin_user_agents 表）。 */