Fix combined packaging stage (#33561)
This pull request updates the Azure Pipelines configuration to
explicitly set the Python version used during the package validation
step. The main change is the addition of a task to ensure Python 3.12 is
available in the build environment before running validation scripts.
Pipeline environment update:
* Added a `UsePythonVersion@0` task in `validate-package.yml` to set up
Python 3.12 before running the package validation script, ensuring
consistent Python environment across builds.
