feat(workflow): 添加更新流程实例变量功能

- 新增 UpdateWorkflowInstanceVariablesCommand 命令处理变量更新逻辑
- 实现 UpdateWorkflowInstanceVariablesCommandHandler 验证并保存变量到数据库
- 添加 UpdateWorkflowInstanceVariablesEndpoint 提供 REST API 接口
- 创建 UpdateWorkflowInstanceVariablesRequest 定义请求数据结构
- 实现 JSON 格式验证确保变量数据有效性
- 支持人工干预修正流程运行时数据的场景需求

src/BuildFlow.Host/Endpoints/Workflow/WorkflowInstance/UpdateWorkflowInstanceVariablesEndpoint.cs

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+using MediatR;
+using Workflow.Application.Features.WorkflowInstances.Commands;
+
+namespace BuildFlow.Host.Endpoints.Workflow.WorkflowInstance;
+
+/// <summary>
+/// 更新单个流程实例的运行时变量。用于人工干预修正流程数据（仅改 Variables，不动状态/定义版本）。
+/// </summary>
+public class UpdateWorkflowInstanceVariablesEndpoint(IMediator mediator)
+    : Endpoint<UpdateWorkflowInstanceVariablesRequest>
+{
+    public override void Configure()
+    {
+        Put("/workflow-instances/{Id}/variables");
+        Permissions("workflow-instance:manage");
+        Summary(s =>
+        {
+            s.Summary = "Update variables of a single workflow instance";
+        });
+    }
+
+    public override async Task HandleAsync(UpdateWorkflowInstanceVariablesRequest req, CancellationToken ct)
+    {
+        var id = Route<Guid>("Id");
+        await mediator.Send(new UpdateWorkflowInstanceVariablesCommand(id, req.Variables), ct);
+        HttpContext.Response.StatusCode = 200;
+        HttpContext.Response.ContentType = "application/json";
+        await HttpContext.Response.WriteAsync("{}", ct);
+    }
+}

src/BuildFlow.Host/Endpoints/Workflow/WorkflowInstance/UpdateWorkflowInstanceVariablesRequest.cs

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+namespace BuildFlow.Host.Endpoints.Workflow.WorkflowInstance;
+
+/// <summary>
+/// 更新流程实例变量请求体。
+/// </summary>
+public class UpdateWorkflowInstanceVariablesRequest
+{
+    /// <summary>新的流程变量 JSON（必须是 JSON 对象，如 {"amount": 5000, "level": 3}）。</summary>
+    public string Variables { get; set; } = string.Empty;
+}

src/modules/workflow/Application/Features/WorkflowInstances/Commands/UpdateWorkflowInstanceVariablesCommand.cs

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+using MediatR;
+
+namespace Workflow.Application.Features.WorkflowInstances.Commands;
+
+/// <summary>
+/// 更新单个流程实例的运行时变量（Variables）命令。
+/// 用于人工干预修正流程数据（如表单填错、条件分支走错），仅改 Variables，不动状态、定义版本、token。
+/// </summary>
+public record UpdateWorkflowInstanceVariablesCommand(
+    Guid InstanceId,
+    string Variables) : IRequest<Unit>;

src/modules/workflow/Application/Features/WorkflowInstances/Handlers/UpdateWorkflowInstanceVariablesCommandHandler.cs

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+using System.Text.Json;
+using MediatR;
+using Workflow.Application.Features.WorkflowInstances.Commands;
+using Workflow.Infrastructure.Persistence;
+
+namespace Workflow.Application.Features.WorkflowInstances.Handlers;
+
+/// <summary>
+/// 更新流程实例变量处理器。校验实例存在且 Variables 为合法 JSON 对象后覆盖写入。
+/// 不校验实例状态：Running/Suspended/Completed 均允许修正变量（人工干预场景）。
+/// </summary>
+public class UpdateWorkflowInstanceVariablesCommandHandler(WorkflowDbContext db)
+    : IRequestHandler<UpdateWorkflowInstanceVariablesCommand, Unit>
+{
+    public async Task<Unit> Handle(UpdateWorkflowInstanceVariablesCommand request, CancellationToken ct)
+    {
+        var instance = await db.WorkflowInstances.FindAsync([request.InstanceId], ct)
+            ?? throw new NotFoundException($"Workflow instance '{request.InstanceId}' not found.");
+
+        // 校验 Variables 是合法 JSON 对象，避免脏数据破坏引擎条件求值（ParseVariables 会吞异常返回空，
+        // 但这里前置校验能给出明确错误而非静默丢失变量）
+        ValidateVariablesJson(request.Variables);
+
+        instance.Variables = request.Variables;
+        await db.SaveChangesAsync(ct);
+
+        return Unit.Value;
+    }
+
+    private static void ValidateVariablesJson(string variables)
+    {
+        try
+        {
+            using var doc = JsonDocument.Parse(variables);
+            if (doc.RootElement.ValueKind != JsonValueKind.Object)
+                throw new BusinessException("流程变量必须是 JSON 对象（如 {\"key\":\"value\"}）。");
+        }
+        catch (JsonException ex)
+        {
+            throw new BusinessException($"流程变量不是合法的 JSON：{ex.Message}");
+        }
+    }
+}