Add Vercel AI Gateway as a first-class channel

[DragonFSKY]

Summary

Add first-class support for Vercel AI Gateway as a multi-protocol upstream channel.

Motivation

Operators can currently approximate parts of Vercel AI Gateway via a custom OpenAI-compatible endpoint, but a dedicated channel gives the console, SDK feature flags, credential validation, routing, and upstream credits checks a documented first-class path.

Scope

• Add vercel channel with default base URL https://ai-gateway.vercel.sh
• Support Vercel AI Gateway's documented OpenAI-compatible APIs: models, chat completions, responses, and embeddings
• Support Vercel AI Gateway's documented Anthropic Messages API
• Use AI Gateway API-key credentials for both OpenAI-compatible and Anthropic-compatible requests
• Expose GET /v1/credits through the existing upstream usage endpoint
• Add console form support and tests