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.

AI Code Review

• Team members only: AI review runs automatically when PR is opened or marked ready for review
• Team members can also trigger a review by commenting @continue-review

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.

---

Summary by cubic

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

^{Written for commit 844bc04. Summary will update on new commits.}

[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.

[cerebrixos]

I rebased this onto current main and reran the fork checks. All source/test/docs checks are green now.

The only remaining red check is the Windows VSIX package job after it successfully builds and uploads both artifacts; the log fails during post-job cache cleanup with:

The operation was canceled.

I do not have permissions to rerun the failed job from the fork, so a maintainer rerun should be enough if the cache cleanup race is transient.