fix(test): mock does_utility_exist in maintenance accept-valid tests

MaintenanceInputValidationAcceptsValidTest asserted batch_process_mock
was called, but the route short-circuits with success=0 (HTTP 200)
when does_utility_exist() returns a missing-binary error. On Windows
CI psql.exe is not at the path pgAdmin probes, so BatchProcess was
never reached and the five 'accepted' scenarios failed with
'AssertionError: False is not true'. Stub does_utility_exist to None
in this test so it focuses on input-validation acceptance, not on the
runtime PostgreSQL binary layout.