feat(js/ai): added registered middleware to allow using it in dev ui

[pavelgj]

ai.defineMiddleware(
  'basic-fallback',
  fallback(ai, {
    models: [googleAI.model('gemini-2.5-flash')],
  })
);

ai.defineFlow('basic-hi-with-fallback', async () => {
  const { text } = await ai.generate({
    model: googleAI.model('gemini-2.5-something-that-does-not-exist'),
    prompt: 'You are a helpful AI assistant named Walt, say hello',
    use: ['basic-fallback'],
  });

  return text;
});

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)