feat(api): add Agents credential and session options (#3967)

## Summary

Adds credential metadata, session options and speech voices, and
corrects the file download response description.

## Changes

- Add metadata to Agents vault credential creation and results; allow
credential updates without replacing authentication.
- Support the ultrafast Agents service tier and report
misalignment_policy_violation for session turns.
- Include fable, onyx and nova in speech voice options.
- Describe file content as binary while preserving legacy
retrieve_content() and its Accept header behavior.

## Compatibility

When constructing beta Agents Credential models yourself, supply
`metadata` (for example, `{}`). Exhaustive handlers should account for
the new service tier and error code.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>