Make nullable schemas referenced with $ref generate optional types

[k-kohey]

Motivation

When a response body schema is referenced with $ref, the generated type is not optional even if the referenced schema is nullable.

components:
  schemas:
    MyObject:
      type: [object, null]
      properties:
        id:
          type: string
      required:
        - id
  responses:
    MyResponse:
      description: OK
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/MyObject'

Current output:

@frozen public enum Body: Sendable, Hashable {
    case json(Components.Schemas.MyObject)
}

Expected output:

@frozen public enum Body: Sendable, Hashable {
    case json(Components.Schemas.MyObject?)
}

Writing the same schema inline already produces ...?, so the behavior differs depending on whether $ref is used. This also contradicts the documented rule in Handling-nullable-schemas.md: "The nullability of a schema is propagated through references."

Modifications

Removed the .reference branch in TypeAssigner.typeUsage(usingNamingHint:withSchema:components:inParent:), which resolved the type name without applying nullability. Every schema now goes through _typeUsage, which already applies withOptional(isOptional(...)) to references, so inline and referenced schemas share a single path.

Request bodies are unaffected, since translateRequestBody explicitly drops optionality with withOptional(false).

Result

Nullable schemas referenced with $ref are generated as optional types. This covers both type: [object, null] and nullable: true, as well as transitive references through another schema. The output for non-nullable references is unchanged.

Test Plan

Added tests to SnippetBasedReferenceTests.