Register hermetic Python 3.12 toolchain

[frankslin]

Use pythons_hub toolchains so Bazel py_test runs with the hermetic 3.12 runtime instead of the system Python.

[BYVoid]

Does Bazel download Python 3.12 if it is not installed?

[Copilot]

Pull request overview

This pull request registers the hermetic Python 3.12 toolchain in Bazel to ensure py_test targets run with a consistent Python runtime instead of relying on the system Python installation.

Changes:

• Added use_repo to expose python_3_12 and pythons_hub repositories from the python extension
• Registered the hermetic Python toolchains using register_toolchains("@pythons_hub//:all")

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[frankslin]

Does Bazel download Python 3.12 if it is not installed?

Yes, it will. When using rules_python with bzlmod and a configured Python toolchain (e.g. python.toolchain(python_version = "3.12")), Bazel downloads a hermetic, prebuilt Python 3.12 runtime and uses it for both py_* rules and pip.parse.

This does not rely on the system Python and works even if Python 3.12 is not installed locally, as long as the toolchain is registered (e.g. via @pythons_hub).