[enc] Expose categories container into Python.

[trivialfis]

• Make pyarrow optional.
• Add a Python class for the container.

Ref #11088

[Copilot]

Pull Request Overview

This PR exposes the categories container into Python by creating a new Python class wrapper and making pyarrow optional for categorical data handling. The changes refactor the categories API to return a container object instead of direct pyarrow arrays, while maintaining backwards compatibility through the export_to_arrow parameter.

Key changes:

• Refactor C API to use opaque handles for category containers with separate memory management
• Add a new Python Categories class that wraps the C++ container and provides controlled access to arrow exports
• Make pyarrow dependency optional by allowing category containers to be created without immediate arrow conversion

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/c_api/c_api.cc	Adds new C API functions for category container creation, export, and cleanup with handle-based memory management
src/data/data.cc	Updates function signature for SparsePage::Push method with type changes
include/xgboost/data.h	Updates header declaration to match implementation changes
python-package/xgboost/core.py	Refactors get_categories methods to return Categories objects and handle optional arrow export
python-package/xgboost/_data_utils.py	Implements new Categories class with handle management and arrow export functionality
python-package/xgboost/_typing.py	Adds new type aliases for arrow category data structures
python-package/xgboost/testing/ordinal.py	Updates all test code to use new Categories API with export_to_arrow parameter

[trivialfis]

cc @rongou .