feat(js/ai): implemented retry and fallback middleware

[pavelgj]

import { retry } from 'genkit/model/middleware';

const { text } = await ai.generate({
  model: googleAI.model('gemini-2.5-pro'),
  prompt: 'You are a helpful AI assistant named Walt, say hello',
  use: [
    retry({
      maxRetries: 2,
      initialDelayMs: 1000,
      backoffFactor: 2,
    }),
  ],
});

import { fallback } from 'genkit/model/middleware';

const { text } = await ai.generate({
  model: googleAI.model('gemini-2.5-pro'),
  prompt: 'You are a helpful AI assistant named Walt, say hello',
  use: [
    fallback(ai, {
      models: [googleAI.model('gemini-2.5-flash')],
      statuses: ['RESOURCE_EXHAUSTED'],
    }),
  ],
});

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)