fix(openai-v2): Add __getattr__ to StreamWrapper to proxy unknown attributes (#4184)

StreamWrapper did not proxy unknown attributes like .headers to the
underlying stream, causing AttributeError when using
with_raw_response.create(stream=True).

Add __getattr__ to delegate attribute lookups to self.stream.

Fixes #4113

Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>