Genkit Go v1.9.0: New models, tool support, and bug fixes

Genkit Go has released version 1.9.0, introducing support for new Google AI models including Imagen 4 and dedicated Gemini TTS models, alongside improvements to tool response mapping and concurrency safety. This update also enhances native constrained generation for tools and adds Claude Opus 4.5 and Llama models via Vertex AI's Model Garden. These changes benefit developers using Genkit for AI applications, particularly those integrating with Google Cloud's AI offerings.