Show story entry point example in story dev tools

### Description

Right now, it's possible to preview a story that does not have the required metadata, but as long as the body of the story looks okay, it's not immediately obvious that the story is actually broken.

This does indeed cause a validation error visible in the "Debug" tab, but for non-technical users using a tool, seeing an HTML attribute name that they've never heard of is not particularly useful.

### Alternatives Considered

There are Google tools that show this for Google's properties (we may even link to them already), but we should have a format-driven way of verifying this that is more immediately obvious that something is wrong.

### Additional Context

_No response_

No files or tests are named. Start by locating the story dev tools and the existing Debug-tab validation for missing metadata; determine where a story entry point example can be shown. Done means a missing required metadata case gives non-technical users an immediately understandable, format-driven indication of the broken story.

Written by the indexing model from the issue text.