[genkit_firebase_ai] Support Vertex AI Gemini API

[goderbauer]

Fixes #295

The Firebase AI Logic SDK supports two Gemini API providers:

• Gemini Developer API, and
• Vertex AI Gemini API

Prior to this change the genkit_firebase_ai package was hardcoding "Gemini Developer API" as API provider. This change enables consumers of the package to optionally use the "Vertex AI Gemini API".

[gemini-code-assist]

Code Review

This pull request introduces a FirebaseAiProvider sealed class to support both Google AI and Vertex AI providers, updating the plugin to instantiate the appropriate Firebase AI client based on the configured provider. The reviewer suggests refactoring the duplicated switch (provider) blocks across the plugin by extracting them into a single private helper getter _firebaseAi to improve code maintainability and reduce duplication.

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