docs(go/plugins/googlegenai): document gemini-3.1 tts model behaviour and add sample

[IzaakGough]

Summary

Document the dedicated Gemini TTS models in the Go googlegenai plugin and add a sample showing how to use them, including the Gemini 3.1 PCM-to-WAV handling needed to produce playable output.

Problem/Root Cause

The Go plugin exposed dedicated Gemini TTS model IDs, but the behavior and usage of those models were not documented. In particular, gemini-3.1-flash-tts-preview returns PCM audio rather than a directly playable WAV file, so users needed sample code showing how to decode the inline media response and wrap it in a WAV container.

Solution/Changes

• Register the dedicated Gemini TTS models with TTS-specific capabilities in the Google AI model registry.
• Add tests covering model classification, declared capabilities, and Google AI-only registration.
• Expand the plugin README with TTS model documentation, usage examples, and a note describing the Gemini 3.1 PCM output behavior.
• Update the Go text-to-speech sample to:
  • return inline media for the Gemini 2.5 TTS flow
  • add a Gemini 3.1 flow that decodes inline PCM audio, converts it to WAV, and returns the generated file path

Testing

• Added go/plugins/googlegenai/tts_test.go covering TTS model registration and capabilities.
• Ran go test ./plugins/googlegenai/...
• Ran go build ./samples/text-to-speech

[gemini-code-assist]

Code Review

This pull request registers and documents new text-to-speech (TTS) models (gemini-2.5-flash-preview-tts, gemini-2.5-pro-preview-tts, and gemini-3.1-flash-tts-preview) in the Google Gen AI plugin, and provides helper functions for converting raw PCM audio to WAV format. Feedback on the changes points out a critical issue in the text-to-speech sample where initializing multiple Genkit instances (g1 and g2) in the same process can cause a panic due to duplicate action registration; instead, a single Genkit instance should be used with the model name specified explicitly in each flow.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.