Widgets: better oneOf schema support

[ddadamhooper]

What does this PR do? What is the motivation?

Supports generating widget documentation once we introduce nested oneOf. See https://github.com/DataDog/datadog-api-spec/pull/5974 -- this failed build

Also: I found it hard to "see" my change in the widget list, so I scratched an itch and changed the "Option 1", "Option 2", etc. to: <type=foo> and <type=bar>. This affects a bunch of documents.

Before	After

How we find a discriminant (e.g., type or data_source):

• In every oneOf branch, the discriminant must be a string enum
• For every discriminant, count unique enum values: i.e., the branch has a single-value enum (a const), and no other branch has a single-value enum with the same value.
• Pick the best discriminant. If there's no perfect match (e.g., FormulaEventsDataSource isn't unique; nor is widget type), ensure the discriminant is good -- i.e., it'll render something useful for most options.

How we render an item (e.g., <type=timeseries>):

• If the discriminant points to a single-value enum, render that
• Otherwise, fallback to what we have today: "Object 3", etc

Merge readiness

• Ready for merge

For Datadog employees:

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AI assistance

I flipped back and forth with Opus 4.8. Most of the code is mine.

Additional notes

[ddadamhooper]

/review

[github-actions]

🤖 Automated review by Claude. AI-generated; verify before acting.

Nice cleanup — replacing generic Option N labels with schema-derived <type=…> names should make the rendered API tables noticeably easier to navigate. A few small nits on the new helpers below; nothing blocking.

_{Reviewed 08c566844f3264a1df15ebbe41e57e449ef66531 — workflow run}