feat: add Modelsell provider

[modelsell]

Description

Adds Modelsell as a named OpenAI-compatible model provider with the fixed API base https://modelsell.com/v1/.

The provider reuses Continue’s existing OpenAI Chat Completions implementation and discovers the model catalog dynamically from GET /v1/models, so no model IDs are hardcoded. This also adds provider documentation and exposes the page in the documentation navigation.

Fixes #13040

Disclosure: I represent Modelsell.

No live authenticated Modelsell API call was performed.

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

Not applicable; this change adds a provider integration and documentation without changing the UI.

Tests

• npm run vitest -- llm/llms/Modelsell.vitest.ts
• npm run tsc:check
• npx eslint llm/llms/Modelsell.ts llm/llms/Modelsell.vitest.ts llm/llms/index.ts
• npx prettier --check core/llm/llms/Modelsell.ts core/llm/llms/Modelsell.vitest.ts core/llm/llms/index.ts docs/customize/model-providers/more/modelsell.mdx docs/docs.json
• JSON parse check for docs/docs.json
• git diff --check

[github-actions]

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 can sign the CLA by just posting a Pull Request Comment same as the below format.

---

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

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}