Make the first event added to a workflow its startup task (#16344)

[RAJEEV2510]

Fixes #16344

Background

A workflow only runs if one of its events is marked as the startup task. With a single event it's obvious which one that is, but you still have to set it by hand, and forgetting it leaves the workflow silently doing nothing (see also #18220, closed in favor of this issue). The issue discussion agreed on both an automatic default and a warning.

Change

• Automatic default (ActivityController.Create): when an event is added and the workflow has no other events, the new activity is saved with IsStart = true. You can still turn it off with the existing start toggle. Tasks and later events are unaffected.
• Warning on save (WorkflowTypeController.Edit POST): if the saved workflow has activities but no startup task, a warning notification is shown next to "Workflow has been saved."
• WorkflowTypeExtensions.IsMissingStartActivity() (Workflows.Abstractions): a single check shared by the save warning and the list badge, with unit tests.
• Workflow list (Views/WorkflowType/Index.cshtml): workflows with activities but no startup task get a "No startup task" badge next to the existing "Disabled" badge.

The editor keeps unsaved canvas changes in session storage. A newly added activity is taken from the server when the editor reloads, so its start flag shows up right away without any client-side script changes.

Testing

• OrchardCore.Workflows builds with no warnings or errors (.NET SDK 10.0.401).
• Not yet checked in a running site. Reviewers:
  1. Create a workflow and add an event (e.g. Content Created). It should show as the startup task (green).
  2. Add a second event. It should not be a startup task.
  3. Turn off the start flag on every event and save. A warning should appear, and the workflow list should show the "No startup task" badge.

[RAJEEV2510]

@Piedone @MikeAlhayek @gvkries, could you please review this when you have a moment?

It adds both parts agreed on in #16344 (and #18220): the first event becomes the startup task automatically, and there's a warning plus a list badge when a workflow has no startup task. CI is green and the branch has no conflicts. Thanks!