feat(js/plugins/google-genai): Support for antigravity agent

[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]

Code Review

This pull request introduces support for the Antigravity model within the Google AI plugin, including the necessary configuration schemas, model definitions, and interaction converters. It also adds unit tests and example flows for the new model. The review feedback highlights an opportunity to improve efficiency by avoiding redundant deep clones when mapping system messages to user messages and suggests adding the AUDIO modality to the configuration schema for consistency with other agents.