Add cancelable async chat streams

[Jaksenc]

Summary

• return cancelable async stream objects from async chat stream APIs instead of bare async generators
• cancel the underlying grpc aio call from aclose()/close()/cancel(), so a consumer task can be stopped while another task is awaiting the next chunk
• preserve telemetry span lifecycle and accumulated streaming responses, including batch streaming behavior
• add a regression test for closing a stream while iteration is blocked

Fixes #142.

Testing

• .venv/bin/python -m ruff format src/xai_sdk/aio/chat.py tests/aio/chat_test.py
• .venv/bin/python -m ruff check src/xai_sdk/aio/chat.py tests/aio/chat_test.py
• .venv/bin/python -m pytest tests/aio/chat_test.py -q
• .venv/bin/pyright --pythonpath .venv/bin/python src/xai_sdk/aio/chat.py
• .venv/bin/pyright --pythonpath .venv/bin/python src/xai_sdk
• git diff --check