Edit Summary label is unclear to users

This issue is important but has changed a lot over the years.

With Drupal 11 and several versions before that we can have had several fields in the same node with a summary each. The teaser metafore is no longer present. However I see that the label "Summary" is not properly connected to it's "parent" field whenever the summary is not closed. Having an option to close the summary also seems to be theme dependant.

This seems to be an accessibility issue as well as in at least some situation the user might not know wether the summary above or directly below the long text field is the on that summarises that specific field.

I guess the Summary label should always have some reference to the main field label.

I would expect this issue belongs to field_ui but is also relevant for a number of themes.

Attaching screenshots from Olivero and Claro.

We discussed this issue at the usability meeting 📌 Drupal Usability Meeting 2024-02-02 Active where we agreed that a simple text change is not a sufficient improvement. At the meeting were present @benjifisher @shaal @ simohell @worldlinemine @duncancm and @rkoller.

We came up with a suggestion to

1. wrap summary and full text into a fieldset
   (we noted that fieldset stlyles may be too sublte for proper accessibility, but that is another issue)
2. use field name as the label for the fieldset
3. label text areas as summary and full text

Summary and trimmed are terms that are used by field display add teaser is a standard view mode that implements displaying the summary and/or trimmed version.

Full text is referenced by the summary description but not present at form.

If we don't want to have the summary field always displayed, we can place it inside a details element. On content creation the details element should be open. On my opinion editing content it should be closed to save space if no summary text was added and open if a summary text was added.

One point that came up in the usability meeting (Comment #26) is that there are use cases for having more than one long-text-with-summary field, or a multi-valued field. I have never seen it, and I do not think it meets the threshold of the 80% use case, but it does come up.

The current link to open the summary form was probably the only way to do it when this form was first designed. That was a long time ago. I think we should use a standard <details> element now that we can.

Probably the summary should be hidden by default, at least when it is empty. That is the least disruptive change from the current behavior.