docs: add Tuning Engines OpenAI-compatible example

[cerebrixos]

Description

Adds Tuning Engines to the OpenAI API-compatible provider docs. The example uses Continue's existing provider: openai plus apiBase configuration path, so this is documentation-only and does not add a new provider enum.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

N/A; docs-only change.

Tests

Not run; docs-only change to a model provider configuration page.

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Summary by cubic

Adds Tuning Engines to the OpenAI‑compatible provider docs. Adds a YAML example using provider: openai with apiBase https://api.tuningengines.com/v1 and a workspace inference key.

^{Written for commit 6dee704. Summary will update on new commits. Review in cubic}

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[cerebrixos]

I have read the CLA Document and I hereby sign the CLA (Venkat Chandra)

[cerebrixos]

I have read the CLA Document and I hereby sign the CLA

[cerebrixos]

recheck

[cerebrixos]

Quick CI note after the author/CLA fix reran checks: this PR changes only docs/customize/model-providers/top-level/openai.mdx. The failing CLI matrix jobs are in src/e2e/headless-anthropic-api-key.test.ts expecting an invalid ANTHROPIC_API_KEY path to exit 1, and the Windows package job appears to complete packaging/upload before being canceled during cache cleanup. I do not see a failure tied to this docs example, but happy to adjust the snippet if maintainers see something specific.