fix(model-runtime): type TTS model voices

[WH-2099]

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Related Issue

Closes #175

Summary

• Added a TTSModelVoice TypedDict for TTS voice options.
• Typed get_tts_model_voices as list[TTSModelVoice] on the runtime protocol and wrapper.
• Updated the TTS dispatch test stub to match the {"name", "value"} voice shape.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation