Add model info for gpt-5.1

[waugustus]

Why are these changes needed?

These changes address the current limitation where GPT-5.1 cannot be used directly within Autogen and enable a seamless upgrade path for users.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[waugustus]

@microsoft-github-policy-service agree