[BUG] Event payload missing when an event bookmark is resumed.

[JaFonz]

Description

As per this issue, the event payload is missing. This fix only applies to events which trigger a workflow, not for events which are mid-workflow and use bookmarks to suspend and resume.

Steps to Reproduce

We are using the Elsa Studio to define the workflows.:

• In one workflow, publish an event with a payload.
• In another workflow, have an event activity which is not a workflow trigger which subscribes to the publish event from the workflow.
• Start the second workflow with the event activity which will run and suspend at the event activity.
• Run the first workflow with the publish activity which will publish the event and complete.
• The second workflow will resume and complete, however the event payload will not be in the event result.

On further investigation, modifying the ExecuteAsync method in Event.cs to add a callback which retrieves the payload from the WorkflowInputMessageInbox resolves this issue:

protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
    {
        var eventName = context.Get(EventName)!;

        if (!context.IsTriggerOfWorkflow())
        {
            var options = new CreateBookmarkArgs
            {
                Stimulus = new EventStimulus(eventName),
                IncludeActivityInstanceId = false,
                Callback = OnResumeAsync
            };
            context.CreateBookmark(options);
            return;
        }

        var input = context.GetWorkflowInput<object?>(EventInputWorkflowInputKey);
        context.SetResult(input);
        await context.CompleteActivityAsync();
    }

    private async ValueTask OnResumeAsync(ActivityExecutionContext context)
    {
        var input = context.GetWorkflowInput<object?>(EventInputWorkflowInputKey);
        context.SetResult(input);
        await context.CompleteActivityAsync();  
    }

I have made the changes above in a fork and am happy to raise a PR if this is acceptable.

[sfmskywalker]

Thanks for the bug report + PR that fixes it!