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.

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<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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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: fs-eire <7679871+fs-eire@users.noreply.github.com>

Author: Copilot

tools/ci_build/build.py

@@ -1027,7 +1027,7 @@ def generate_build_tree(
         if not (
             args.build_shared_lib
             and is_windows()
-            and args.cmake_generator == "Visual Studio 17 2022"
+            and args.cmake_generator in ("Visual Studio 17 2022", "Visual Studio 18 2026")
             and args.use_full_protobuf
         ):
             raise BuildError("Fuzz test has only be tested with build shared libs option using MSVC on windows")

tools/ci_build/build_args.py

@@ -174,6 +174,7 @@ def add_cmake_build_config_args(parser: argparse.ArgumentParser) -> None:
             "NMake Makefiles JOM",
             "Unix Makefiles",
             "Visual Studio 17 2022",
+            "Visual Studio 18 2026",
             "Xcode",
         ],
         default=None,  # Will be set later based on OS and WASM