Add audio feature tests for OpenAI provider

[sumaiazaman]

Summary

Adds missing audio feature tests for the OpenAI provider. OpenAiGateway implements generateAudio() but had no corresponding feature test.

Tests cover:

• Request includes model, input, response format, and posts to the correct endpoint
• Default-female voice alias resolves to alloy
• Default-male voice alias resolves to ash
• Custom voice ID is passed through unchanged
• Instructions are included in the request when provided
• Response is base64-encoded with audio/mpeg mime type and correct meta
• Request sends bearer token authorization

[pushpak1300]

Thanks.