feat(go/plugins/googlegenai): register gemma-4 models for Google AI

[adesinah]

This PR registers gemma-4 models for Google AI. This addition went through a series of tests.

Closes: #5466

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 Gemma models (gemma-4-26b-a4b-it and gemma-4-31b-it) in the Google GenAI plugin. It defines a new GemmaSupports configuration specifying capabilities unique to Gemma models (such as no system role or tool calling) and registers these models for Google AI. Additionally, a new test suite is added to verify the classification, capabilities, and registration of these models. I have no feedback to provide as there are no review comments.

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