Bring back ngspice on mac

[g-braeunlich]

Follow up of: #242

At the moment ngspice is not tested for mac.

• tests/test_problem_implementations.py, test_python_problem_impl:

  if problem_class.__module__.startswith("engibench.problems.power_electronics") and sys.platform == "darwin":
      pytest.skip(f"Skipping {problem_class.__name__} on MacOs")

• Installation of ngspice is outcommented in .github/workflows/test.yml for mac

Goal: Follow the strategy suggested by @SoheylM :

• short-term CI fix: pin the runner to a macOS image that still has archived Homebrew bottles for ngspice 44.2 / libngspice 44.2, and install those exact archived bottles directly
• durable fix: add NGSPICE_PATH support in EngiBench and publish our own pinned macOS ngspice 44.2 tarballs via GitHub Releases, so we stop depending on Homebrew for this one binary

Additional comment:

There is no clean way to pin the ngspice version via homebrew. For that, we need to either pin to macos-15 (which is pretty old and is a shaky fix), or - best long term solution - host our own image of ngspice 44.2 and use that.

[linear]

SOH-120