docs: include dto comments in openapi schemas

Tiku.Api/Contracts/HealthDtos.cs

@@ -1,5 +1,11 @@
 namespace Tiku.Api.Contracts;
 
+/// <summary>
+/// 健康检查响应。
+/// </summary>
+/// <param name="Status">服务状态，例如 ok。</param>
+/// <param name="Service">服务名称。</param>
+/// <param name="CheckedAt">检查时间。</param>
 public sealed record HealthResponseDto(
     string Status,
     string Service,