OPCUA-3461 Add generator-selection invariant test

[parasxos]

What

A fourth hermetic invariants test, .CI/test_generator_selection.py, guarding the CMake generator-selection contract of generateCmake.py at the argv level — plus wiring into both invariants lanes (alma9 + WS2025).

Why

The OPCUA-3360 fix removed the dead hardcoded Visual Studio 15 2017 Win64 generator: with no env set, CMake now auto-detects the installed Visual Studio. But every Windows CI build job exports QUASAR_CMAKE_GENERATOR=Ninja, and the BE-ICS w2025 provisioning (quasar-team/images) sets no generator env at all — so the no-env auto-detect path, the exact path a developer on a provisioned box hits out of the box, had zero CI coverage. A regression there would ship green and surface only on BE-ICS machines.

The 6 cases

Forced OS	Setup	Must produce
Windows	nothing set	no -G (auto-detect — the BE-ICS out-of-box path)
Windows	QUASAR_CMAKE_GENERATOR=Ninja	-G Ninja (the CI path)
Windows	generator + QUASAR_CMAKE_GENERATOR_PLATFORM=x64	-G … -A x64
Windows	_PLATFORM alone	no -G, no -A
Linux	default	no -G
Linux	--builder Ninja	-G Ninja

Every case additionally asserts the dead Visual Studio 15 2017 string never reappears.

Hermetic in the style of the existing three invariants: the module graph is stubbed, subprocessWithImprovedErrors captures the argv instead of executing — no CMake, no compiler, any host OS. Mutation-tested: re-introducing the old hardcode fails 4 of 6 cases, exit 1.

Also

The WS2025 invariants step ran all tests as bare pwsh lines — a step fails only on the last command's exit code, so an early test failure could be masked by a later pass. Each test now gets an explicit exit-code check (same chk pattern the SDK build step uses).

[jimmat92]

This also contains another important fix. Without chk() earlier scripts could fail and the job succeed.
You can separate this to another commit but not necessary in my opinion.

[jimmat92]

The tempfile.mkdtemp() dirs created on run_case() are not cleared. In test_no_unbound_delegate.py there is try/finally and shutil.rmtree(tmp, ignore_errors=True) which clears the generated dirs.
Worth to add this for consistency on run_case()