Test flake in `test/test_errors.py::test_async_client_timeout`

[stefanvanburen]

https://github.com/connectrpc/connect-python/actions/runs/19037701195/job/54366251891#step:8:78

Seen this once or twice; would be great to track it down.

[stefanvanburen]

Slightly different error in the same test in https://github.com/connectrpc/connect-python/actions/runs/19037701195/job/54367710405#step:8:83.