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/config/providers.js

@@ -129,7 +129,8 @@ export const PROVIDERS = {
   },
   "vercel-ai-gateway": {
     baseUrl: "https://ai-gateway.vercel.sh/v1/chat/completions",
-    format: "openai"
+    format: "openai",
+    retry: { 429: 2 }
   },
   glm: {
     baseUrl: "https://api.z.ai/api/anthropic/v1/messages",