docs: add Tuning Engines OpenAI-compatible setup

[cerebrixos]

Summary

• add Tuning Engines to the available chat models guide as an OpenAI-compatible gateway
• show how to use the existing @aigne/openai integration with a Tuning Engines inference key and baseURL
• clarify that AIGNE keeps ownership of agents, skills, workflows, and runtime behavior

Why

We are an AI infrastructure team using AIGNE with Tuning Engines. Since the guide already lists multiple model providers and the OpenAI adapter supports baseURL, this gives users a copy-paste path for governed model access, policy checks, traces, audit logs, and usage/cost reporting. It would mean a lot to us if this is useful for AIGNE users, and I am happy to adjust the wording or placement.

Validation

• git diff --check
• checked @aigne/openai OpenAIChatModel options for baseURL support

[cerebrixos]

Hello team @wangshijun . Please do consider this PR for merge. So much appreciate in advance :)