List the discriminator property first in API union variants

[akira28]

Each oneOf/anyOf variant in the API reference now lists the property that selects it first, so the choice is the first thing its fields say.

Affects: API reference

Prompt summary: Fix elastic/docs-eng-team#1073. The discriminator parameter of a oneOf/anyOf variant should be detected and listed first, because it is the quickest way to tell variants apart.

Why

A variant lists its fields in schema order. The property that carries the choice, often type, can sit last among ten fields. A reader has to scan the whole list to find what distinguishes the variant.

What

Discriminator first in each variant

A variant moves its selecting property to the top of its field list. The property comes from the OpenAPI discriminator.propertyName when the schema declares one. Otherwise it is the property that every variant fixes to its own single enum value, the same rule the variant labels already use.

Order of other fields

All other fields keep their schema order. A union with no declared discriminator and no property fixed per variant lists fields as before.

Verify

dotnet test tests/Elastic.ApiExplorer.Tests/
# BuildPropertyList_VariantSelectedByProperty_ListsThatPropertyFirst: declared and enum-detected cases

Check on the preview

• Kibana: Create output — the type field of each variant was 10 to 35 rows down; it now opens each variant.
• Kibana: Create a case — seven variants with type fourth; it now comes first.
• Kibana: Add a case comment — two variants with type third and fifth; it now comes first.

[github-actions]

Docs preview (local build)

Handbook preview: https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/4490/