Let API union variants fold their fields behind a toggle

[akira28]

Variants of a oneOf/anyOf behind chips now have a "hide properties" toggle, the same one object rows use, so a reader can shorten the view.

Affects: API reference

Prompt summary: Fix elastic/docs-eng-team#1074. Objects fold their fields but oneOf/anyOf blocks do not, and nothing says how to collapse them. Add a way to collapse them and keep the default as it is.

Why

Two or more variants show behind chips, and the picked variant lists its fields at once. Those fields have no toggle, so a long variant cannot be folded. Object rows next to it fold with "hide properties".

What

Toggle on chip variants

A variant behind chips renders the same "hide properties" toggle as a lone variant or an object row. The existing disclosure script handles the click, and links or find-in-page into a folded variant still open it.

Default stays expanded

The variant starts expanded, as before: picking a chip is the choice to read that variant. Only the control to fold it is new.

Verify

dotnet test tests/Elastic.ApiExplorer.Tests/
# Render_ThreeVariants_ShowsTheFieldsAtOnceWithAToggleAndWithoutRepeatingTheName

Check on the preview

• Kibana: Create a case — open the type variants and click "hide properties" under a chip; the fields fold and "show properties" brings them back.
• Kibana: Create output — a long variant (35 fields) folds to a short view.