configure python sessions to use ppm instead of pypi

[ian-flores]

https://positpbc.slack.com/archives/C05K993PVEV/p1744650015473229

this would be helpful for a consistent experience in defaulting to package manager for all languages, rather than just R

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Migrated from rstudio/ptd#1802

[ian-flores]

Work was started on this in PR #52 (branch python-ppm-config). The PR adds PythonPackageManagerUrl to SiteSpec, injects PIP_INDEX_URL into Workbench sessions via launcher-env DCF config, and includes 3 unit tests. However, we're not going to pursue this further at this time. Closing the associated draft PR — the branch is preserved for future reference.