Fix wrong use of $ref and nullable

[11010cy]

Composed schemas $ref and nullable are not supported. And inline composed schemas has bug using tooling.

Per the openapi spec, properties adjacent to refs are ignored:
https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#reference-object
This object cannot be extended with additional properties and any properties added SHALL be ignored. except for summary and description.

dag_run_timeout:
              nullable: true
              $ref: '#/components/schemas/TimeDelta'

dag_run_timeout:
 oneOf:
  - type: null
  - $ref: '#/components/schemas/TimeDelta'

All the above definitions are not correct.

related issues:
OpenAPITools/openapi-generator/issues/11352
apache/airflow-client-python/issues/40

[airflow]
related prs
pr:apache/airflow/pull/21080