docs: add Tuning Engines custom host example

[cerebrixos]

Summary

• add a Tuning Engines example to the existing OpenAI-compatible providers section
• show Configuration with host, basePath, environment API key, and relaxed parsing
• keep the change limited to documentation

Why

We are an AI infrastructure team using this Swift SDK with Tuning Engines. Since the README already documents custom hosts and OpenAI-compatible providers, this adds a concrete setup for teams that want governed model access, policy checks, traces, audit logs, and usage/cost reporting. It would mean a lot to us if this is useful, and I am happy to adjust or shorten the wording.

Validation

• git diff --check
• checked OpenAI.Configuration supports host, basePath, and parsingOptions