feat(js/plugins/google-genai): Added support for gemini-3-flash-preview

[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 gemini-3-flash-preview model into the Google GenAI plugin, enhancing its capabilities. It also refines the thinkingLevel configuration by adding a MINIMAL option and updates the test applications to showcase the distinct features of the new Flash model, ensuring proper functionality and demonstrating its usage.

Highlights

• New Model Support: Added gemini-3-flash-preview to the list of known Gemini models for both Google AI and Vertex AI plugins, enabling its use within the system.
• Expanded Thinking Levels: Introduced a MINIMAL option to the thinkingLevel configuration enum, providing more granular control over the model's reasoning process for Gemini 3.0 models.
• Updated Test Applications: Modified test flows to specifically demonstrate the gemini-3-flash-preview model and its unique thinkingLevel options, distinguishing it from other Gemini 3.0 models and ensuring proper functionality.

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[gemini-code-assist]

Code Review

The pull request introduces support for the gemini-3-flash-preview model by adding it to the known models and expanding the thinkingLevel enum to include MINIMAL in both Google AI and Vertex AI configurations. It refactors existing thinking-level flows into thinking-level-pro (supporting LOW and HIGH thinking levels) and adds new thinking-level-flash flows that demonstrate the gemini-3-flash-preview model with MINIMAL, LOW, MEDIUM, and HIGH thinking levels using a specific logic puzzle prompt. The review comments point out that this logic puzzle prompt is duplicated across the new thinking-level-flash flows and suggest extracting it into a shared constant for improved maintainability.