windows: build with Visual Studio 2026 / newer MSVC toolsets

[exzile]

Fixes #4333.

The Windows build scripts hardcode Visual Studio 2022 Build Tools and a specific MSVC toolset version, so a machine with only Visual Studio 2026 (v18) cannot build OVMS. This PR detects the installed Visual Studio and toolset, and fixes a couple of newer-toolchain breakages it surfaced.

Changes

1. Auto-detect Visual Studio via vswhere (windows_install_build_dependencies.bat, windows_build.bat, windows_setupvars.bat, windows_test.bat)

• Replaces the hardcoded C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools with vswhere -latest -requires ...VC.Tools.x86.x64 -property installationPath (works with BuildTools/Community/Pro/Enterprise).
• Derives the CMake generator (Visual Studio <major> <year>) from the detected major version instead of the literal "Visual Studio 17 2022".
• Auto-detects the installed MSVC toolset version instead of the pinned 14.44.35207.

2. Allow newer MSVC toolsets to compile XLA (.bazelrc)

• MSVC 14.51+ (VS 2026) raises C5285 ("specializing std::is_signed is forbidden") on TensorFlow/XLA's quantized-type specializations (xla/tsl/framework/type_traits.h). XLA builds with /WX, so it becomes a fatal C2220. Added build:windows --copt=/wd5285 (+ --host_copt) to suppress this benign upstream warning. Without it, only the older 14.44 toolset builds.

3. OpenCV generator default toolset / CMake 4.x (windows_install_build_dependencies.bat)

• Dropped the hardcoded -T v142 (VS 2019 toolset, absent on a VS 2026 box); cmake uses the generator's default (installed) toolset.
• Pass -D PYTHON3_EXECUTABLE and disable the OpenCV python bindings — newer CMake (4.x) errors in OpenCV's python auto-detection (find_package called with invalid argument "OFF"); OVMS only needs the C++ libs.

4. Non-privileged GenAI link (windows_install_build_dependencies.bat)

• mklink /d → mklink /j for the openvino link. A directory junction is functionally equivalent here and does not require SeCreateSymbolicLinkPrivilege (admin / Developer Mode), so the dependency install works in a non-elevated shell.

Testing

• Built ovms.exe end-to-end on Visual Studio 2026 (18.7, MSVC 14.51) with windows_install_build_dependencies.bat + windows_build.bat opt --with_python — produces a working server (OpenVINO Model Server 2026.3.0).
• Not re-tested on VS 2022 (no VS2022 box available); the vswhere/generator/toolset detection resolves the 2022 values by the same logic, and the /wd5285 + OpenCV changes are no-ops on the older toolset.

Notes

• CMake must be on PATH (the VS "C++ CMake tools" component or a standalone install).