Improve handling of non-HTTP context in HttpEndpoint

Updated the HttpEndpoint class to handle non-HTTP contexts better. Instead of throwing an exception when it finds itself in a non-HTTP context, it now creates a bookmark that allows the invoker to save the state and resume execution from there. A callback has also been added to the bookmarks creation process to prepare for resumption actions.

src/modules/Elsa.Http/Activities/HttpEndpoint.cs

@@ -166,7 +166,7 @@ protected override async ValueTask ExecuteAsync(ActivityExecutionContext context
 
         if (!context.IsTriggerOfWorkflow())
         {
-            context.CreateBookmarks(GetBookmarkPayloads(context.ExpressionExecutionContext), includeActivityInstanceId: false);
+            context.CreateBookmarks(GetBookmarkPayloads(context.ExpressionExecutionContext), includeActivityInstanceId: false, callback: OnResumeAsync);
             return;
         }
 
@@ -191,8 +191,10 @@ private async ValueTask OnResumeAsync(ActivityExecutionContext context)
 
         if (httpContext == null)
         {
-            // We're not in an HTTP context, so let's fail.
-            throw new Exception("Cannot execute in a non-HTTP context");
+            // We're executing in a non-HTTP context (e.g. in a virtual actor).
+            // Create a bookmark to allow the invoker to export the state and resume execution from there.
+            context.CreateBookmark(OnResumeAsync, BookmarkMetadata.HttpCrossBoundary);
+            return;
         }
 
         await HandleRequestAsync(context, httpContext);