Link to Issue or Description of Change

Issue 5937: #5937

Problem:
Went through a quick start and experienced the following error:
errorCode : "ServerError" errorMessage : "503 UNAVAILABLE. {'error': {'code': 503, 'message': 'This model is currently experiencing high demand. Spikes in demand are usually temporary. Please try again later.', 'status': 'UNAVAILABLE'}}"

Solution:
. Updated adk-python/README.md
Change the Quick Start "Agent" example to use the Gemini model class with retry_options instead of a bare model string.

2. Updated adk-python/contributing/samples/core/quickstart/agent.py
   Similarly update the quickstart sample to use Gemini with retry_options so it matches the README and serves as a working reference.

Testing Plan

Unit Tests:

• [Y] All unit tests pass locally.
  Manual End-to-End (E2E) Tests:
  Just follow the changed quick start guidelines, even if the model is in high demand, now it should be able to automatically retry and avoid transient 503 errors.

Checklist

• [ Y] I have read the CONTRIBUTING.md document.
• [ Y] I have performed a self-review of my own code.
• [ Y] I have commented my code, particularly in hard-to-understand areas.
• [ Y] I have added tests that prove my fix is effective or that my feature works.
• [ Y] New and existing unit tests pass locally with my changes.
• [ Y] I have manually tested my changes end-to-end.
• [ Y] Any dependent changes have been merged and published in downstream modules.

Additional context