Add _async_soft_close method to AsyncAdapt_pyathena_cursor

SQLAlchemy 2.0.46 calls cursor._async_soft_close() in the async result
handling path (_ensure_sync_result). Add a no-op implementation matching
the pattern used by SQLAlchemy's own asyncpg adapter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: laughingman7743

pyathena/aio/sqlalchemy/base.py

@@ -93,6 +93,9 @@ def fetchall(self) -> Any:
     def setinputsizes(self, sizes: Any) -> None:
         self._cursor.setinputsizes(sizes)
 
+    async def _async_soft_close(self) -> None:
+        return
+
     # PyAthena-specific methods used by AthenaDialect reflection
     def list_databases(self, *args: Any, **kwargs: Any) -> Any:
         return await_only(self._cursor.list_databases(*args, **kwargs))