feat: 实现租户列表查询接口

- 获取租户列表：GET /api/admin/v1/tenants
- 用于填充租户下拉选择器

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Domain/TakeoutSaaS.Domain/Tenants/Repositories/ITenantRepository.cs

@@ -22,4 +22,12 @@ public interface ITenantRepository
     /// <param name="cancellationToken">取消标记。</param>
     /// <returns>租户列表（仅返回找到的租户）。</returns>
     Task<IReadOnlyList<Tenant>> FindByIdsAsync(IReadOnlyCollection<long> tenantIds, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// 获取所有租户列表（用于下拉选择器）。
+    /// </summary>
+    /// <param name="keyword">关键字（租户名称/编码）。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>租户列表。</returns>
+    Task<IReadOnlyList<Tenant>> GetAllAsync(string? keyword, CancellationToken cancellationToken = default);
 }