[enc] Expose the categories container to C.

[trivialfis]

No description provided.

[Copilot]

Pull Request Overview

This PR moves experimental category-related API functions from hidden/internal status to the public C API header, making them officially available as of version 3.2.

Key changes:

• Moved CategoriesHandle typedef from implementation file to public header
• Relocated comprehensive documentation for XGDMatrixGetCategories, XGDMatrixGetCategoriesExportToArrow, and XGBCategoriesFree from implementation to header
• Added public declarations for XGBoosterGetCategories and XGBoosterGetCategoriesExportToArrow with proper documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/c_api/c_api.cc	Removed documentation comments for category functions that were moved to the public header, cleaned up internal comments
include/xgboost/c_api.h	Added CategoriesHandle typedef, comprehensive documentation for all category-related functions with @since 3.2 tags and experimental API notes

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[hcho3]

Can you give me some more context on this PR? I thought the Booster object already supported exporting the categories container as Arrow arrays? How does this PR change things?

[trivialfis]

Can you give me some more context on this PR?

I hid the C functions in the last release in case of unexpected issues. It took me a while to figure out all the states, like whether one needs to re-code data during training continuation, and when input is a DMatrix. This PR exposes these functions to the C header, but they are still marked as experimental.