Fix `model_validator` deprecation warning in `SimvueConfiguration`

[kzscisoft]

Description of Bug

When running the Python API the check_valid_server validator of type mode="after" prints a deprecation warning due to after validators being required to return and act on Self.

Steps To Reproduce

• Identified during execution of the Simvue CLI tests.

Expected behavior

No deprecation warning displayed.

Actual Behaviour

Warning appears in Pytest output saying this behaviour will be deprecated.

Setup

• OS: Ubuntu 25.10
• Python 3.13.4