Make async_sandbox_factory teardown exception-safe

Use asyncio.gather(..., return_exceptions=True) so a failure killing
one sandbox doesn't skip cleanup of the others, matching the
independence the previous per-finalizer approach provided.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: mishushakov

python/tests/conftest.py

@@ -1,3 +1,4 @@
+import asyncio
 import os
 
 import pytest
@@ -58,8 +59,9 @@ async def factory(*, template_name: str = template, **kwargs):
 
     yield factory
 
-    for sandbox in sandboxes:
-        await sandbox.kill()
+    await asyncio.gather(
+        *(sandbox.kill() for sandbox in sandboxes), return_exceptions=True
+    )
 
 
 @pytest.fixture