feat: expose vector_stores on client.beta

[Moondy217]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Expose vector_stores under client.beta to enable Assistant file_search + vector_store_ids support.

Additional context & links

Currently, OpenAI Python SDK 1.78.0 does not expose client.beta.vector_stores.
This PR shows how to enable the missing entry point using:

@cached_property
def vector_stores(self) -> VectorStores:
    from ..vector_stores import VectorStores
    return VectorStores(self._client)

See API reference: https://platform.openai.com/docs/assistants/tools/file-search
Related issue: #

[RobertCraigie]

Thanks for the PR! But those docs were outdated, looks like they've since been fixed.