feat: expose async stream cancel handle

[MukundaKatta]

Summary

• return a cancelable async iterator handle from chat.stream() while preserving existing async for usage
• expose cancel() and aclose() so callers can stop the underlying gRPC stream early
• document the early-stop pattern and add a regression test for transport cancellation

Closes #142

Tests

• uv run pytest tests/aio/chat_test.py -q
• uv run pytest tests/aio/chat_test.py::test_stream_cancel_cancels_underlying_rpc tests/aio/chat_test.py::test_streaming tests/aio/chat_test.py::test_stream_creates_span_with_correct_attributes -q
• uv run ruff check src/xai_sdk/aio/chat.py tests/aio/chat_test.py
• uv run pyright src/xai_sdk/aio/chat.py tests/aio/chat_test.py
• uv run ruff format --check src/xai_sdk/aio/chat.py tests/aio/chat_test.py
• git diff --check