Update tests for pydantic v2: `validate_command`

[jluethi]

When we switch over to depend on fractal-tasks-core 1.2.0, we should also update the tests (especially validate_command in test_valid_task_interface) to use the new pydantic V2 logic.

Additionally, validate_command currently triggers the following pre-commit issue for me:

tests/test_valid_task_interface.py:17:14: UP022 Prefer `capture_output` over sending `stdout` and `stderr` to `PIPE`

We should decide whether we want UP022 to be checked. If yes, we should fix the test. If no, we should add it to the exclusion list