Add aclose alias to AsyncStream

[LittleChenLiya]

Summary

Add an aclose() alias to AsyncStream for compatibility with async stream consumers that expect the standard close method name.

This keeps AsyncStream aligned with other async response wrappers in the SDK and avoids AttributeError in integrations that call await stream.aclose().

Changes

• add AsyncStream.aclose() as an alias for close()
• add regression tests covering the alias and confirming it closes the underlying response

Testing

• python -m pytest tests/test_streaming.py -k "async_stream_has_aclose_alias or async_stream_aclose_closes_response" -n 0

Closes #2853