fix: default provider to openai, follow redirects on callback

- Change default provider from "google" to "openai" to match default
  model "gpt-5.4" (was causing Google API 404)
- Add follow_redirects=True to callback httpx client to handle
  morphllm.com → www.morphllm.com redirects

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>