Require content templates to have at least one dynamic property source

I don't really see why this is something we would have to enforce. It's totally fine if users want to test static templates as content templates before they map props to a dynamic prop source.

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📌 Move `PropSourceEndpointTest` into new `XbConfigEntityHttpApiTest::testComponent()` Active landed a while ago.

Lots of other statements in the issue summary are no longer true because many issues ended up going in a different direction based on design changes. Let's get the issue summary updated.

EDIT: cross-posted

I don't really see why this is something we would have to enforce. It's totally fine if users want to test static templates as content templates before they map props to a dynamic prop source.

I strongly disagree.

A ContentTemplate that is either empty or contains only StaticPropSources no longer is a template for presenting structured data. It's the equivalent of a spreadsheet where a formula (~content template) applied to different columns (~content entities aka structured data) all renders the exact same output, and ignores the inputs. In other words: it's nonsense.

It's fine for me to be in such a state while working on it (IOW: auto-saves like this are fine, and allow you to preview it as such!), but it never makes sense to go live with such a ContentTemplate (IOW: validation constraint needed, which is checked when going from auto-saved to actually saved).

I don't see how this is a data integrity concern? This seems like a UX concern and us trying to babysit the users. I feel very strongly that we should not introduce validation of this type. It should be up to the user to decide what kind of template they want to create. Maybe all of the data is fetched using JSON:API or maybe you just want to preview how a static template would look like? This isn't really changing from what you can do today either because you can create an empty display mode today and that's fine.