assistants: fallback type in discriminated unions

[kristapratico]

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

Changes being requested

Provides a well-defined behavior / fallback type for response data that doesn't fit any known type in a discriminated union. Aims to handle more gracefully the scenario in which a client may be using a version of the library that does not yet know about a tool type / its response.

Additional context & links

https://github.com/openai/openai-python/issues/1574