Fix win arm64 release.

[trivialfis]

• The aarch64 version was only released for the xgboost-cpu, but not the xgboost package. This PR fixes that.
• Add a simple test for the xgboost Win ARM64 package.

At the moment, the xgboost and xgboost-cpu packages should be identical. I'm keeping both packages to avoid confusing existing users and to preserve the opportunity to support ARM-based GPU devices.

[Copilot]

Pull request overview

This PR fixes the Windows ARM64 Python wheel release for the xgboost PyPI package (previously only the xgboost-cpu variant was released for aarch64 on Windows) and adds CI coverage to validate the produced Win ARM64 wheel.

Changes:

• Include win_arm64 in the release artifact downloader so Win ARM64 wheels are fetched for release packaging.
• Add a Win ARM64 wheel test job that downloads the built wheel artifact, installs it, and runs a basic Python test.
• Adjust Conda test environments and update installation documentation to reflect platform support changes.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ops/script/release_artifacts.py	Adds win_arm64 to the list of platforms when downloading Python wheel artifacts.
ops/conda_env/win64_test.yml	Tweaks Windows test environment dependencies (CUDA + pip-installed py-ubjson).
ops/conda_env/win64_cpu_test.yml	Introduces a CPU-focused Windows test Conda environment for wheel validation.
ops/conda_env/macos_cpu_test.yml	Adjusts macOS CPU test environment dependencies (moves py-ubjson to pip section).
doc/install.rst	Updates platform support documentation and wheel capability table.
.github/workflows/windows.yml	Renames the Windows CUDA test job identifier.
.github/workflows/python_wheels_winarm64.yml	Uploads the Win ARM64 xgboost wheel as an artifact and adds a job to test installing/running it.

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