feat(vercel-ai-gateway): support embeddings, images and credit usage

Extend Vercel AI Gateway beyond chat: add OpenAI-compatible embeddings
and image generation endpoints, credit balance fetch on the usage
dashboard, retry on 429, and models catalog fetcher.

Thinking/reasoning mapping is omitted pending a project-wide refactor.

Co-authored-by: Ngô Tấn Tài <tantai@newnol.io.vn>
Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/handlers/embeddingProviders/index.js

@@ -6,6 +6,7 @@ import openaiCompatNode from "./openaiCompatNode.js";
 const OPENAI_COMPAT_PROVIDERS = [
   "openai", "openrouter", "mistral", "voyage-ai", "fireworks",
   "together", "nebius", "github", "nvidia", "jina-ai",
+  "vercel-ai-gateway",
 ];
 
 const ADAPTERS = {