[Feat]: Add support for other LLMs

[lgrosales]

Is your feature request related to a problem? Please describe.

The AP2 project primarily supports the "gemini-2.5-flash" model. This is evident from several Python agent definitions in the repo (such as RetryingLlmAgent and usage in root_agent and subagents), where "model="gemini-2.5-flash"" is specified as the LLM in use. The code constructs the LLM client with genai.Client() and makes content requests using the Gemini API.

Describe the solution you'd like

I'd like for the AP2 protocol implementation reference to be LLM agnostic.

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[TeamCyberCrypto]

genai.GenerativeModel("gemini-1.5-flash") = gemini-2.5-flash