`prompt_cache_retention` enum variant does not match docs

[willbicks]

The current openapi schema describes the two valid enum variants for prompt_cache_retention as in-memory and 24h:

        prompt_cache_retention:
          anyOf:
            - type: string
              enum:
                - in-memory
                - 24h
              description: >
                The retention policy for the prompt cache. Set to `24h` to enable extended prompt caching,
                which keeps cached prefixes active for longer, up to a maximum of 24 hours. [Learn
                more](https://platform.openai.com/docs/guides/prompt-caching#prompt-cache-retention).
            - type: 'null'

While the docs specify the valid options as 24h and in_memory (note the underscore instead of the dash) at https://platform.openai.com/docs/guides/prompt-caching#configure-per-request:

If you don’t specify a retention policy, the default is in_memory. Allowed values are in_memory and 24h.