* fix(models): scope the OpenAI-compat rules to BaseChatOpenAI, not a class-path allowlist
* address review: drop redundant stream_usage helper, close test matrix
- Remove _enable_stream_usage_by_default and its now-unused _OPENAI_COMPAT_USE_PATHS
tuple. The class-field stream_usage fallback already sets stream_usage=True for
every BaseChatOpenAI subclass (they all declare the field), so the helper's
use-path allowlist gated nothing real — verified a no-op in prod, and the two
stream_usage tests stay green on main with the helper present. Those tests used a
BaseChatModel stub that does not declare the field; point them at a real ChatOpenAI
capturing class so they exercise the fallback they now depend on.
- Give the non-OpenAI normalization-skip test an actual api_base value so it exercises
the skip path (api_base passed through verbatim, never rewritten to base_url).
- Add an Unreleased CHANGELOG entry for the api_base behavior change on the five
affected subclasses.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
