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/Application/TakeoutSaaS.Application/App/Tenants/Contracts/TenantListItemDto.cs

@@ -0,0 +1,37 @@
+using System.Text.Json.Serialization;
+using TakeoutSaaS.Domain.Tenants.Enums;
+using TakeoutSaaS.Shared.Abstractions.Serialization;
+
+namespace TakeoutSaaS.Application.App.Tenants.Contracts;
+
+/// <summary>
+/// 租户列表项 DTO（用于下拉选择器）。
+/// </summary>
+public sealed class TenantListItemDto
+{
+    /// <summary>
+    /// 租户 ID（雪花，序列化为字符串）。
+    /// </summary>
+    [JsonConverter(typeof(SnowflakeIdJsonConverter))]
+    public long Id { get; init; }
+
+    /// <summary>
+    /// 租户编码。
+    /// </summary>
+    public string Code { get; init; } = string.Empty;
+
+    /// <summary>
+    /// 租户名称。
+    /// </summary>
+    public string Name { get; init; } = string.Empty;
+
+    /// <summary>
+    /// 租户简称。
+    /// </summary>
+    public string? ShortName { get; init; }
+
+    /// <summary>
+    /// 租户状态。
+    /// </summary>
+    public TenantStatus Status { get; init; }
+}

src/Application/TakeoutSaaS.Application/App/Tenants/Handlers/ListTenantsQueryHandler.cs

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+using MediatR;
+using TakeoutSaaS.Application.App.Tenants.Contracts;
+using TakeoutSaaS.Application.App.Tenants.Queries;
+using TakeoutSaaS.Domain.Tenants.Repositories;
+
+namespace TakeoutSaaS.Application.App.Tenants.Handlers;
+
+/// <summary>
+/// 获取租户列表查询处理器。
+/// </summary>
+public sealed class ListTenantsQueryHandler(ITenantRepository tenantRepository)
+    : IRequestHandler<ListTenantsQuery, IReadOnlyList<TenantListItemDto>>
+{
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<TenantListItemDto>> Handle(ListTenantsQuery request, CancellationToken cancellationToken)
+    {
+        // 1. 查询租户列表
+        var tenants = await tenantRepository.GetAllAsync(request.Keyword, cancellationToken);
+
+        // 2. 映射 DTO
+        return tenants.Select(t => new TenantListItemDto
+        {
+            Id = t.Id,
+            Code = t.Code,
+            Name = t.Name,
+            ShortName = t.ShortName,
+            Status = t.Status
+        }).ToArray();
+    }
+}

src/Application/TakeoutSaaS.Application/App/Tenants/Queries/ListTenantsQuery.cs

@@ -0,0 +1,15 @@
+using MediatR;
+using TakeoutSaaS.Application.App.Tenants.Contracts;
+
+namespace TakeoutSaaS.Application.App.Tenants.Queries;
+
+/// <summary>
+/// 获取租户列表查询（用于下拉选择器）。
+/// </summary>
+public sealed record ListTenantsQuery : IRequest<IReadOnlyList<TenantListItemDto>>
+{
+    /// <summary>
+    /// 关键字（租户名称/编码）。
+    /// </summary>
+    public string? Keyword { get; init; }
+}