Add Tuning Engines model provider sample

[cerebrixos]

Summary

• add a Tuning Engines OpenAI-compatible model provider sample for OpenAI Agents SDK workflows
• add worker and workflow runner scripts using TUNING_ENGINES_API_KEY, TUNING_ENGINES_MODEL, and TUNING_ENGINES_BASE_URL
• update the model providers README with setup instructions

Validation

• git diff --check
• python3 -m compileall openai_agents/model_providers

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}