Models: Don't wrap anyOf, oneOf, allOf in curly brackets

Demonstration API definition

openapi: 3.0.0
info:
  title: Test API
  version: 0.0.1
servers:
  - url: 'localhost:3000/api'
    description: Local API URL.
paths:
  /test:
    get:
      requestBody:
        content:
          application/json:
            schema: {}

      responses:
        '200':
          description: OK
components:
  schemas:
    Sample:
      oneOf:
        - type: integer
        - type: boolean

    SampleObject:
      type: object
      properties:
        foo:
          type: integer

Expected Behavior

When rendering Models don't wrap anyOf, allOf, or oneOf in {}

Current Behavior

Context

Wrapping oneOf (etc) in { } is in conflict with using { } to represent an Object. It implies one of the models is embedded within an Object.

This is especially evident when the oneOf element is collapsed:

Start with the provided OpenAPI 3.0 YAML example and compare the current and expected Model rendering shown in the issue images. Trace the Model renderer responsible for oneOf, anyOf, and allOf; done means these combinators render without curly brackets while ordinary objects retain them.