refactor(test): drop redundant pytestmark asyncio

pyproject asyncio_mode=auto already covers async test dispatch; the
explicit pytestmark was a vestige.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-v-b

tests/test_coalesce.py

@@ -23,8 +23,6 @@
 if TYPE_CHECKING:
     from collections.abc import AsyncIterator, Callable, Sequence
 
-pytestmark = pytest.mark.asyncio
-
 
 def _buf(data: bytes) -> Buffer:
     return default_buffer_prototype().buffer.from_bytes(data)