fix(test): add pytest-asyncio for async test support

Add pytest-asyncio==0.26.0 to core dev requirements and configure
asyncio_mode=auto in pyproject.toml to fix failing async tests in
service_test.py.

pyproject.toml

@@ -16,3 +16,4 @@ pythonpath = [
     "src/translation"
 ]
 testpaths = ["test"]
+asyncio_mode = "auto"

src/core/requirements.dev.txt

@@ -1 +1,2 @@
 black==25.1.0
+pytest-asyncio==0.26.0