Fix race condition in WebHooks integration specs causing UnknownReactorId

The WebhookReactor is a kernel-side system reactor registered asynchronously
via ReactorsReactor processing EventStoreAdded. On early test contexts, the
DiscoverAndRegister call had not yet completed when GetHandlerById was invoked,
causing UnknownReactorId to be thrown.

Retry GetHandlerById with a 30-second timeout (polling every 100ms) in the
shared Register() helper so all contexts wait for the reactor to become
available. Also remove the duplicate GetHandlerById + WaitTillReachesEventSequenceNumber
calls in with_headers_and_subscribed_event_type_is_appended, which were already
performed inside Register().

Author: einari

Integration/DotNET.InProcess/for_Webhooks/given/webhook_states_after_registering.cs

@@ -19,7 +19,24 @@ protected async Task Register(params (WebhookId, WebhookTargetUrl, Action<IWebho
             await Webhooks.Register(id, targetUrl, configure);
         }
 
-        var webhookReactor = EventStore.Reactors.GetHandlerById("$system.Cratis.Chronicle.Observation.Webhooks.WebhookReactor");
+        // The WebhookReactor is a kernel-side system reactor registered asynchronously
+        // via ReactorsReactor processing EventStoreAdded. Retry until it is available.
+        IReactorHandler? webhookReactor = null;
+        var deadline = DateTimeOffset.UtcNow.AddSeconds(30);
+        while (DateTimeOffset.UtcNow < deadline)
+        {
+            try
+            {
+                webhookReactor = EventStore.Reactors.GetHandlerById("$system.Cratis.Chronicle.Observation.Webhooks.WebhookReactor");
+                break;
+            }
+            catch (UnknownReactorId)
+            {
+                await Task.Delay(100);
+            }
+        }
+
+        webhookReactor.ShouldNotBeNull();
         await webhookReactor.WaitTillReachesEventSequenceNumber(EventSequenceNumber.First);
 
         StoredWebhooks = await EventStoreStorage.Webhooks.GetAll();

Integration/DotNET.InProcess/for_Webhooks/when_registering/with_headers_and_subscribed_event_type_is_appended.cs

@@ -1,9 +1,7 @@
 // Copyright (c) Cratis. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-using Cratis.Chronicle.Events;
 using Cratis.Chronicle.EventSequences;
-using Cratis.Chronicle.Reactors;
 using Cratis.Chronicle.Webhooks;
 using context = Cratis.Chronicle.InProcess.Integration.for_Webhooks.when_registering.with_headers_and_subscribed_event_type_is_appended.context;
 
@@ -21,9 +19,6 @@ public async Task Establish()
         {
             await Register(
                 (WebhookId, TargetUrl, builder => builder.WithEventType<SomeEvent>().WithHeader("some-header", "some-value")));
-
-            var webhookReactor = EventStore.Reactors.GetHandlerById("$system.Cratis.Chronicle.Observation.Webhooks.WebhookReactor");
-            await webhookReactor.WaitTillReachesEventSequenceNumber(EventSequenceNumber.First);
         }
 
         public async Task Because()