Drop the bu-3 and bu-3-max model ids everywhere they were surfaced:
- ChatBrowserUse no longer lists them as valid bu-* aliases (provider-prefixed
ids like openai/gpt-5.5 are still accepted by the gateway).
- Remove their custom pricing entries and the README pricing blocks.
- Update docstrings, README quickstart, and the beta_agent example to use
openai/gpt-5.5 as the default, with bu-2-0 shown as a commented alternative.
- Drop the tests that asserted bu-3/bu-3-max acceptance and pricing.
Saurav Panda