fix(domain): correlate withdrawal by process instance to the subscribed execution (refs #12)

verify-stack surfaced a Flowable 500: delivering messageEventReceived to the Beoordelen task's

execution is wrong — a message boundary event's subscription lives on its own execution. Correlate

instead by the registration's process instance id (recorded at submit): query the execution

subscribed to RegistratieIngetrokken and deliver the message there. Withdrawal moves from

IUserTaskClient to IWorkflowClient.WithdrawProcessAsync; the handler no longer needs a task lookup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -66,7 +66,7 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
             "service/query/tasks", request, ct);
 
         var tasks = page?.Data ?? [];
-        return [.. tasks.Select(t => new BeoordelingTask(t.Id, t.ExecutionId, RegistrationId.Parse(t.RegistrationId())))];
+        return [.. tasks.Select(t => new BeoordelingTask(t.Id, RegistrationId.Parse(t.RegistrationId())))];
     }
 
     public async Task ClaimAsync(string taskId, string behandelaar, CancellationToken ct = default)
@@ -86,18 +86,35 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
         response.EnsureSuccessStatusCode();
     }
 
-    public async Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default)
+    public async Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default)
     {
-        // Deliver the withdrawal message to the Beoordelen task's execution, tripping the process's
-        // interrupting message boundary event so the registratie instance ends (ADR-0014). Flowable
-        // takes messageEventReceived as a PUT on the subscribed execution.
-        var request = new MessageEventRequest("messageEventReceived", IngetrokkenMessage);
+        // Correlate the withdrawal message to the instance: find the execution subscribed to it (the
+        // interrupting message event's own execution — NOT the user task's), then deliver
+        // messageEventReceived to that execution so the process ends (ADR-0014). If nothing is
+        // subscribed (the process is not parked at Beoordelen) this is a best-effort no-op.
+        var subscribed = await GetAsync<ExecutionQueryResult>(
+            $"service/runtime/executions?messageEventSubscriptionName={IngetrokkenMessage}&processInstanceId={processInstanceId}",
+            ct);
 
+        var execution = subscribed?.Data?.FirstOrDefault();
+        if (execution is null)
+            return;
+
+        var request = new MessageEventRequest("messageEventReceived", IngetrokkenMessage);
         using var response = await SendAsync(
-            $"service/runtime/executions/{executionId}", request, ct, HttpMethod.Put);
+            $"service/runtime/executions/{execution.Id}", request, ct, HttpMethod.Put);
         response.EnsureSuccessStatusCode();
     }
 
+    private async Task<TResponse?> GetAsync<TResponse>(string path, CancellationToken ct)
+    {
+        var message = new HttpRequestMessage(HttpMethod.Get, new Uri(options.BaseUrl, path));
+        message.Headers.Authorization = new AuthenticationHeaderValue("Basic", BasicCredentials());
+        using var response = await http.SendAsync(message, ct);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadFromJsonAsync<TResponse>(ct);
+    }
+
     private async Task<TResponse?> PostAsync<TRequest, TResponse>(string path, TRequest body, CancellationToken ct)
     {
         using var response = await SendAsync(path, body, ct);
@@ -154,7 +171,6 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
 
     private sealed record UserTaskDto(
         [property: JsonPropertyName("id")] string Id,
-        [property: JsonPropertyName("executionId")] string ExecutionId,
         // Flowable's task-query returns the (included) process variables under "variables", not
         // "processVariables"; the request opts in via includeProcessVariables.
         [property: JsonPropertyName("variables")] IReadOnlyList<Variable>? Variables)
@@ -172,6 +188,11 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
 
     private sealed record ProcessInstance([property: JsonPropertyName("id")] string Id);
 
+    private sealed record ExecutionQueryResult(
+        [property: JsonPropertyName("data")] IReadOnlyList<ExecutionDto>? Data);
+
+    private sealed record ExecutionDto([property: JsonPropertyName("id")] string Id);
+
     private sealed record AcquiredJob(
         [property: JsonPropertyName("id")] string Id,
         [property: JsonPropertyName("variables")] IReadOnlyList<Variable> Variables)