docs(firebase_ai): add App Check integration example to README

[vansh949]

Description

The firebase_ai README doesn't show how to use it alongside Firebase App Check,
even though App Check is recommended for securing Gemini API calls in production apps.
Added a new section with a short explanation and a working code example showing how to
pass FirebaseAppCheck.instance to FirebaseAI.instanceFor().

Related Issues

Closes #17200

Checklist

• I've reviewed the contributing guidelines
• No breaking changes
• Documentation only change

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[SelaseKay]

Hi @vansh949, thanks for the attempt. The sample is currently incorrect for firebase_ai: FirebaseAI.instanceFor() does not exist. The available constructors are FirebaseAI.googleAI() and FirebaseAI.vertexAI().

Also, the appCheck parameter is currently deprecated in source because Firebase AI now auto-discovers registered App Check services. That said, I'll go ahead and close this.