Fall back to the CUDA_PATH toolkit when the preferred CUDA version is not installed

[Fedr]

platform.props hard-selects the CUDA version per platform toolset (v142 → 11.4, v143 → 12.0, v145 → 13.2). If a user doesn't have exactly that version, the build fails with MSB4019 on the CUDA $(MRCudaVersion).props import even when a perfectly usable newer toolkit is installed.

This PR adds an evaluation-time fallback: when the selected version's MSBuild integration is absent from the running Visual Studio, MRCudaVersion is re-pointed to the toolkit referenced by the CUDA_PATH environment variable (the NVIDIA installer keeps it at the latest installed version), provided that version's VS integration is present. So a machine with only CUDA 13.3 installed now builds with 13.3 instead of erroring.

Details:

• The hard mapping stays the preference: when the mapped version is installed (as in CI, which installs it explicitly), nothing changes. This avoids silently switching toolkits on machines that have several CUDA versions installed.
• The check runs at evaluation time (property functions + Exists()), which is required because the CUDA props import itself happens during evaluation — a target-based detection would run too late.
• MRCudaGencode (Build MSBuild CUDA for the same GPU architectures as CMake #6359) picks up the fallback version automatically via its version-range conditions, e.g. a 13.x fallback gets the 75/86/89/120 architecture set.
• If neither the mapped version nor the CUDA_PATH version has its integration installed (or CUDA_PATH is unset), the behavior is exactly as before: MSB4019 naming the missing props file.
• Also covers the case of a future toolset with no mapping yet (MRCudaVersion empty): the CUDA_PATH version is used if its integration is installed.

Verified locally: with the mapped version present the selection is unchanged (12.0 for v143); with the mapping temporarily pointed at a non-installed 12.9 and CUDA_PATH=v13.2, evaluation selects 13.2 and MRCuda compiles end-to-end with the CUDA 13 architecture set; with CUDA_PATH pointing at a toolkit without VS integration (or unset), the original MSB4019 remains.

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