Add Visual Studio 2026 CMake generator support (#26802)
### 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:**
```bash
./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.
<!-- START COPILOT CODING AGENT SUFFIX -->
<!-- START COPILOT ORIGINAL PROMPT -->
<details>
<summary>Original prompt</summary>
> 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.
</details>
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