feat(js/plugins/google-genai/vertexai): Added Virtual Try On model

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @ifielker, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates support for the new Vertex AI Virtual Try On model into the Google GenAI plugin. It introduces a dedicated configuration schema and request conversion logic for the try-on functionality, allowing users to leverage this advanced image generation capability. The changes ensure that try-on specific requests are correctly processed and that the new model is properly registered and testable within the existing framework.

Highlights

• New Virtual Try On Model Support: Introduced a new ImagenTryOnConfigSchema to define the configuration parameters specific to the Virtual Try On model, including personImage, productImages, and various generation settings.
• Dynamic Request Conversion: Modified the toImagenPredictRequest function to intelligently detect and route requests for the Virtual Try On model to a newly added toImagenTryOnRequest converter, ensuring proper handling of try-on specific parameters.
• Model Registration: The new virtual-try-on-preview-08-04 model has been registered within the KNOWN_MODELS list, making it discoverable and usable within the system, complete with its specific capabilities and configuration schema.
• Example Usage: Added a new example flow (imagen-try-on) in the test application to demonstrate how to use the Virtual Try On model with base64 encoded person and product images.
• Comprehensive Testing: New unit tests have been added for both the converters.ts and imagen.ts files to validate the correct functionality and schema association of the Virtual Try On model and its request conversion logic.

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[gemini-code-assist]

Code Review

This pull request adds support for the Virtual Try-On model in the Vertex AI plugin. The changes include a new configuration schema, updates to request converters, and a new known model definition. I've identified a few areas for improvement, primarily around TypeScript type safety, which are critical for maintaining a robust and predictable public API. There are also opportunities to refine the new Zod schema for better validation and to improve the clarity of the implementation in the converters and test application.