[typescript-fetch] Preserve nullable primitive oneOf branches

[simPod]

Context

A nullable primitive branch in an OpenAPI 3.0 oneOf loses null in the generated TypeScript type. This affects both the separate-model and withoutRuntimeChecks output modes.

For example:

oneOf:
  - type: string
    nullable: true
  - type: number

The generated declaration is number | string, although this schema also accepts null.

Root cause

The oneOf type-name strings do not retain branch nullability. The corresponding composed-schema properties still contain it.

Decision

Count nullable branches from the composed-schema properties and add null to the generated type union when exactly one branch is nullable. This fixes the type metadata without a template override or a new generator option.

Consequences

Nullable primitive unions include null. Unions with zero or two nullable primitive branches keep their non-null declaration, consistent with the exactly-one matching rule of oneOf. Nullable array elements do not make the array itself nullable.

cc @joscha

---

Summary by cubic

Fixes the TypeScript fetch generator so nullable primitive oneOf branches preserve null in the generated union type.

• Adds null to the union only when exactly one branch is nullable, matching the oneOf semantics.
• Nullable array items don't make the containing array itself nullable.

^{Written for commit 80e2602. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}