Fix MSBuild property for VS2019+ Current layout and instance-derived install dir

[CAOShurong]

Fixes #5275

Summary

The EnvironmentInfo.MSBuild property has two independent defects on VS2019+ machines:

1. Wrong layout since VS2019. For every vs_ver >= 15.0 the property builds paths as MSBuild\<vs_ver>\bin, but since Visual Studio 2019 (v16) the on-disk layout is the version-independent MSBuild\Current\bin. The issue's reported VS2026 output (MSBuild\18.7\bin) and my local VS2022 output (MSBuild\17.1\bin) are both nonexistent directories produced by this branch.
2. Lossy version formatting. vs_ver is a major.minor float, so a real 17.12 install formats as "17.1". That string is used both as the registry lookup key (self.ri.lookup(self.ri.vs, f'{self.vs_ver:0.1f}')) — which finds nothing — and in the default path. VSInstallDir therefore resolves to a directory that does not exist.

Fix

• Use MSBuild\Current\bin (+ Roslyn) for vs_ver >= 16.0; keep the historical per-version layouts for VS2015 (C:\Program Files (x86)\MSBuild\14.0\bin, which never had the VS-directory prefix) and VS2017 (MSBuild\15.x\bin).
• Filter returned candidates to directories that actually exist when at least one candidate exists, so consumers no longer receive dead paths.
• Prefer the exact installationPath recorded by the Visual Studio installer instance state (SystemInfo.known_vs_paths, already parsed from C:\ProgramData\Microsoft\VisualStudio\Packages\_Instances\*\state.json) in SystemInfo.VSInstallDir, matching on major version. This avoids the lossy float formatting entirely.

Validation

On this Windows 11 machine with VS2022 Community 17.12 installed:

before:
  VSInstallDir: C:\Program Files (x86)\Microsoft Visual Studio 17.1   # does not exist
  MSBuild: ['...\Microsoft Visual Studio 17.1\MSBuild\17.1\bin',          # does not exist
            '...\Microsoft Visual Studio 17.1\MSBuild\17.1\bin\Roslyn']  # does not exist

after:
  VSInstallDir: C:\Program Files\Microsoft Visual Studio\2022\Community
  MSBuild: ['C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\bin',
            'C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\bin\Roslyn']

Both post-fix paths contain MSBuild.exe / Roslyn binaries.

Tests: new setuptools/tests/test_msvc_msbuild.py (8 cases) covering VS2015/VS2017/VS2019/VS2022-multiline/VS2026 layouts, empty-result behavior for old versions, existence filtering, plus an opt-in real-environment check asserting every returned path exists when a genuine VS install is present. Red/green verified: 5 of the new cases fail on unpatched main, all 8 pass with the fix; existing setuptools/_distutils/compilers/C/tests/test_msvc.py suite still green (15 passed, 1 skipped).

Signed-off-by: Shurong Cao 35021@users.noreply.github.com

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[Avasam]

Also done in #5276 and #5278