chore(js/plugins/vertexai): add gemini-2.5-pro-exp-03-25 model

[nozomi-koborinai]

Add support for Vertex AI's Gemini 2.5 Pro Experimental model (03-25)

This model is already available in the Vertex AI API and has been added to other language implementations (go/googleai, go/vertexai, js/googleai) but was missing from the JS implementation.

How to use

import { genkit } from 'genkit';
import { vertexAI, gemini25ProExp0325 } from '@genkit-ai/vertexai';

// Configure Genkit with the Vertex AI plugin
const ai = genkit({
  plugins: [vertexAI({ location: 'us-central1' })],
});

// Use the model directly
await ai.generate({
  model: gemini25ProExp0325,
  prompt: 'Explain quantum computing in simple terms'
});

This addition ensures our JS SDK stays in sync with the latest available Vertex AI models.

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)