Rate limit for Gemini fails silently

[azhang]

open-codex doesn't show any erorrs when hitting 429
DEBUG=true open-codex doesn't show error message though it does show 429 code

The default Gemini model is pro-preview-03-25 which doesn't have a free tier so I imagine many people may run into this issue silently where it just says "thinking..." forever while hitting the gemini api repeatedly.

curl "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-pro-preview-03-25:generateContent?key=
returns

{
  "error": {
    "code": 429,
    "message": "Gemini 2.5 Pro Preview doesn't have a free quota tier. Please use Gemini 2.5 Pro Experimental (models/gemini-2.5-pro-exp-03-25) instead. For more information on this error, head to: https://ai.google.dev/gemini-api/docs/rate-limits.",
...