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/openai.js

@@ -12,6 +12,7 @@ const ENDPOINTS = {
   github: "https://models.github.ai/inference/embeddings",
   nvidia: "https://integrate.api.nvidia.com/v1/embeddings",
   "jina-ai": "https://api.jina.ai/v1/embeddings",
+  "vercel-ai-gateway": "https://ai-gateway.vercel.sh/v1/embeddings",
 };
 
 export default function createOpenAIEmbeddingAdapter(providerId) {