Handle required-only allOf constraints

[darkbasic]

Summary

Handle allOf members that only make existing object properties required.

Fixes #1474
Related to #1520 and #2570

Problem

Given a schema such as:

allOf:
  - $ref: "#/components/schemas/Base"
  - type: object
    required: [name]

openapi-typescript generated Base & Record<string, never>. The second member is a presence constraint, but it was transformed as an empty object, making the resulting type effectively unusable.

Solution

Exact { required: [...] } and { type: object, required: [...] } members are translated through the existing parent required flow. For referenced schemas this produces WithRequired<Base, "name"> instead of an empty-object intersection.

The implementation is intentionally conservative:

• Every required key must exist in the retained object schemas.
• Typed constraints require another retained, non-nullable object assertion.
• Schemas with extra keywords, callbacks, renamed or filtered properties, unions, nullability, or early-return types keep the previous behavior.
• Recursive allOf references are followed with cycle protection.
• Existing discriminator handling is preserved while allowing additional non-discriminator fields to become required.

The main logic is in transform/schema-object.ts. The composition tests include generated TypeScript compilation checks, and the DigitalOcean fixture was regenerated because it contains several real examples of this schema pattern.

Validation

• 324 package tests passed
• TypeScript and generated-example checks passed
• Build and package export validation passed
• DigitalOcean generated fixture compiles

Notes

While the original implementation has been written by me, after several rounds of validation by GPT-5.6 Sol it found so many corner cases that it basically rewrote it from scratch.

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