Handle invalid CMake skip compiler test values

[LunaMeerkats]

Summary

• route invalid non-string cmake_skip_compiler_test values through Meson's existing configuration error
• add a regression for the reported boolean value

Fixes #13391.

Root cause

Properties.get_cmake_skip_compiler_test() asserted that the raw property was a
string before its enum conversion. That prevented boolean input from reaching
the existing ValueError handler, so Meson exposed an internal
AssertionError instead of a normal EnvironmentException.

The documented values (always, never, and dep_only) are unchanged.

Validation

• regression observed failing with AssertionError before the fix
• focused regression: 1 passed
• complete InternalTests class: 70 passed, 2 skipped
• python run_format_tests.py
• python -m compileall -q mesonbuild unittests/internaltests.py
• python -m flake8 mesonbuild
• python -m pylint mesonbuild/envconfig.py
• valid/default enum behavior and invalid boolean/integer/list inputs checked directly

A broader Windows run completed with 478 passed and 194 skipped; its 15 failures
were in unrelated platform/toolchain cases (including C#/Rust discovery and
Windows path handling). Repository-wide Pylint and MyPy were also limited by
pre-existing Python 3.14 deprecations and POSIX-only attributes on Windows.

AI assistance

OpenAI Codex assisted with issue triage, implementation, regression coverage,
validation, and diff review. The final two-file diff and recorded test evidence
were reviewed before submission.

[LunaMeerkats]

CI follow-up: I inspected all five failed jobs before making any code changes.

• The three MSYS2 jobs completed their unit-test phases successfully, then each failed later in the unrelated common/230 external project integration case. The same case fails in all three corresponding base-commit MSYS2 jobs.
• The Windows unit-test phase completed with 437 passed / 246 skipped; the later project-test failures are common/216 custom target input extracted objects, native/7 selfbuilt custom, and native/8 external program shebang parsing. The base-commit Windows workflow is also red in an unrelated custom-target test.
• The macOS job's only failure is LinuxlikeTests.test_pkgconfig_gen; the base-commit macOS workflow is likewise red in pkg-config coverage.
• The other 25 checks passed, including format, Flake8, MyPy, Pylint, Linux, Cygwin, cross-only, and Azure jobs.

None of the failure output points to get_cmake_skip_compiler_test() or the new regression, which remains green locally. I have therefore left the patch unchanged.