Fix async service cleanup in LocalBackend.close()

src/art/local/backend.py

@@ -1,3 +1,4 @@
+import asyncio
 import json
 import math
 import os
@@ -90,7 +91,26 @@ def __exit__(
         exc: BaseException | None,
         tb: TracebackType | None,
     ) -> None:
-        self._close()
+        try:
+            # If an event loop is running, force users to use the async context manager
+            asyncio.get_running_loop()
+            raise RuntimeError(
+                "LocalBackend used with 'with' inside a running event loop. Use 'async with LocalBackend()' instead."
+            )
+        except RuntimeError:
+            # No event loop running; safe to close synchronously
+            self._close()
+
+    async def __aenter__(self) -> Self:
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc: BaseException | None,
+        tb: TracebackType | None,
+    ) -> None:
+        await self.close()
 
     async def close(self) -> None:
         """