docs: document preserve-one-of-in-all-of OpenAPI setting

[devin-ai-integration]

Summary

preserve-one-of-in-all-of is a new opt-in OpenAPI spec setting. Today, a oneOf (or anyOf) nested inside an allOf gets flattened: all variants' properties are merged into one object and marked optional, so the union disappears and invalid property combinations become representable. With the setting enabled, the importer keeps the union, so generated SDKs and the API Reference reflect the spec's real constraints (and the API Reference shows a variant selector).

Implements docs for: feat(cli): add preserve-one-of-in-all-of OpenAPI setting (fern-api/fern#17289)

Adds one <ParamField> to the shared OpenAPI spec settings snippet (fern/snippets/openapi-specs.mdx, rendered on the generators.yml references for SDKs and API definitions), placed next to the other oneOf/allOf settings:

• default false
• before/after shape: allOf: [oneOf: [A, B, C], S] becomes oneOf: [A & S, B & S, C & S], each variant keeping its own required properties plus the shared ones
• caveat: enabling it changes generated request/response shapes in every SDK and renders a variant selector in the API Reference
• union members that declare a discriminator are left intact

No new page and no changelog entry: the CLI changelog for 5.86.0 already covers the release (fern/products/cli-api-reference/cli-changelog/2026-07-30.mdx), and this is a CLI/SDK setting rather than a Docs product feature.

Link to Devin session: https://app.devin.ai/sessions/11132b3c32464381be946f33f0af06fd

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