refactor: migrate exec bridge to anyio and harden cancellation (#6)

tests/conftest.py

@@ -1,4 +1,11 @@
 import sys
 from pathlib import Path
 
+import pytest
+
 sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
+
+
+@pytest.fixture
+def anyio_backend() -> str:
+    return "asyncio"