feat(google-vertex): Add Gemini TTS speech model support

[kyushun]

Background

Google Vertex AI now supports Gemini TTS models for text-to-speech generation. This PR adds support for these speech models to the @ai-sdk/google-vertex provider.

Summary

Added support for Gemini TTS models (gemini-2.5-flash-tts, gemini-2.5-pro-tts)

Manual Verification

I've added examples in examples/ai-core/src/generate-speech/google-vertex* and have run them.

Checklist

• [x] Tests have been added / updated (for bug fixes / features)
• [x] Documentation has been added / updated (for bug fixes / features)
• [x] A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• [x] Formatting issues have been fixed (run pnpm prettier-fix in the project root)
• [x] I have reviewed this pull request (self-review)

Related Issues

#6945

[valenradovich]

qq: will this PR add support for Chirp 3 HD voices from Google as well?

[kyushun]

@valenradovich Probably, but I’m not familiar enough with non-Gemini TTS models to confirm. This PR is scoped to Gemini TTS only.

[Devoxer]

Hi, thanks for your efforts, is there any date for when to expect this merge ? and is it tested yet with Chirp 3 ? I really want this SDK to be a one stop shop for all AI implementations needed for my apps

[gr2m]

We are looking into it, one thing we want to make sure is that the options look the same across providers.

Thank you for the great PR!

[songkeys]

i might be wrong but it seems google ai studio also has the tts feature. this seems not vertex-specific. should we move this into google provider package?