Cloudflare AI Gateway and Vercel AI Gateway model providers

Two gateway providers are available. Cloudflare reaches many upstream providers and Cloudflare’s own Workers AI models through one account token and account ID. Vercel AI Gateway uses its OpenAI-compatible Responses endpoint and syncs its model list.
The Cloudflare provider lists Workers AI text-generation models with their context window and tool support. Third-party model IDs on Cloudflare are still added by hand, because no endpoint lists what an account can bill.
This also fixes a token dropped from Cloudflare streams when the model emitted a purely numeric token.
See Cloudflare and Vercel. PRs #3996, #4021.