Generate pylibcudf stubs with stubgen-pyx

[vyasr]

Summary

• add a local pre-commit hook that regenerates tracked public pylibcudf .pyi files with stubgen-pyx==0.2.18
• add a wrapper script to scope generation to the existing public pylibcudf stub set and preserve SPDX headers
• replace the handwritten pylibcudf stubs with generated output for review

Motivation

We have been improving stubgen-pyx upstream, and this draft gives cudf developers a concrete diff for evaluating whether generated pylibcudf stubs are close enough to replace the handwritten files.

Notes for reviewers

This is intentionally a draft/evaluation PR. The generated stubs are much more complete and source-derived than the handwritten versions, but there are still visible gaps worth reviewing:

• CudaStreamLike is lost in generated signatures, generally becoming object | None
• some curated public helper aliases and protocols disappear in favor of lower-level generated imports
• some generated signatures expose implementation details or private helpers that the handwritten stubs hid
• full mypy currently reports generated-stub issues and downstream precision regressions

Validation

• deleted tracked python/pylibcudf/pylibcudf/**/*.pyi files and regenerated them with pre-commit run stubgen-pyx-pylibcudf --all-files
• reran pre-commit run stubgen-pyx-pylibcudf --all-files after rebasing on latest upstream/main; it passed with no further changes
• pre-commit run ruff --files ci/checks/generate_pylibcudf_stubs.py .pre-commit-config.yaml
• pre-commit run ruff-format --files ci/checks/generate_pylibcudf_stubs.py .pre-commit-config.yaml
• pre-commit run yamllint --files .pre-commit-config.yaml
• pre-commit run verify-copyright --files ...
• git diff --check

Full commit-time mypy was also run while preparing this branch and currently fails on the generated stubs; that failure is part of the review signal for this draft.

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