[Docs] Note third-party gateway availability for Gemini 4 Argon

[zoomote]

​Opened on behalf of @taltas. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

Follow-up to the merged #66. Adds one "Other providers" bullet to the Gemini 4 Argon section of docs/providers/gemini.md: the model is also available through OpenRouter (google/gemini-4-argon), Vercel AI Gateway (google/gemini-4-argon), and Requesty (vertex/gemini-4-argon), and Zoo Code fetches those provider catalogs automatically.

Why this change was made

A provider re-audit (requested by a maintainer via Discord) confirmed these three gateways now list Gemini 4 Argon in their live catalogs. The paired Zoo-Code follow-up PR (Zoo-Code-Org/Zoo-Code#2073) fixes wrong context/output limits that the Vercel AI Gateway catalog reports for this model.

Impact

Users can see at a glance which third-party providers carry Gemini 4 Argon, without implying the Gemini API is the only option. No structural docs changes.

pnpm run check-types passes. Availability was verified against the live OpenRouter, Vercel AI Gateway, and Requesty model catalogs, and the page was rendered locally in the Docusaurus dev server (see screenshot).

Screenshots

Related PRs

• Zoo-Code follow-up: [Fix] Clamp Gemini 4 Argon limits to Google's values on Vercel AI Gateway Zoo-Code#2073
• Original registration (merged): [Feat] Add Gemini 4 Argon to Gemini and Vertex AI providers Zoo-Code#2043 and [Docs] Document Gemini 4 Argon on Gemini and Vertex AI provider pages #66