Add Support for OpenAI GPT-4.1 Model Family

[Tafkas]

Describe the need of your request

ProxyAI currently lacks support for OpenAI's latest GPT-4.1 model family (GPT-4.1, GPT-4.1 mini, and GPT-4.1 nano).

Proposed solution

Integrate all three GPT-4.1 model variants into ProxyAI.

Additional context

Technical documentation references:
GPT-4.1 Documentation
GPT-4.1 Mini Documentation
GPT-4.1 Nano Documentation

[carlrobertoh]

Keeping this ticket open until I add them for the OpenAI provider as well.

[somedev7]

Keeping this ticket open until I add them for the OpenAI provider as well.

I did it for me locally, and I could prepare PRs to llm-client and to this repo if you agree =)

And also could add gemini 2.5 pro preview.