Add Visual Studio 2026 CMake generator support

[Copilot]

Description

Add "Visual Studio 18 2026" to the list of supported CMake generators in the build script, matching the CMake 3.32+ release that added VS 2026 support.

Changes:

• Added "Visual Studio 18 2026" to --cmake_generator choices in build_args.py
• Updated fuzz testing generator validation to accept both VS 2022 and VS 2026

Usage:

./build.py --cmake_generator "Visual Studio 18 2026" --build_dir build

Motivation and Context

CMake 3.32+ includes Visual Studio 18 2026 generator support. ONNX Runtime's build script only supported up to Visual Studio 17 2022, blocking users with VS 2026 installations.

Original prompt

Recently, Visual Studio 2026 is released and CMake has the support for it: https://cmake.org/cmake/help/latest/generator/Visual%20Studio%2018%202026.html

However, ONNX Runtime's build script (build.py) does not have the support for it. currently supported cmake generators are:

build.py --help

...
--cmake_generator {MinGW Makefiles,Ninja,NMake Makefiles,NMake Makefiles JOM,Unix Makefiles,Visual Studio 17 2022,Xcode}
...

Please make a PR to add support for the latest visual studio.

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[fs-eire]

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