python: reclassify base uv interpreters as Global

[austin3dickey]

Fixes #14912. Reclassifies the uv base interpreters to Global.

This aligns their treatment in the interpreter picker, recommendation ranking, (upcoming) health check, and venv-base selection with every other standalone Python, and gives downstream code a clean way to distinguish externally-managed interpreters from virtual environments.

uv-created virtual environments are unchanged; they stay classified as (uv: parent_dir).

Release Notes

New Features

• N/A

Bug Fixes

• Classify uv-managed standalone Python installs as global interpreters instead of uv environments (python: classify standalone uv Python installs as "Global", not "uv" #14912)

Validation Steps

To verify manually:

1. Install a uv-managed standalone Python that is not on your $PATH:

   uv python install 3.12

2. Create a uv virtual environment somewhere so both kinds are present:

   uv venv /tmp/uv-demo-venv

3. Cold-start Positron and open the interpreter picker; wait for discovery to finish.
4. Confirm the uv-managed standalone Python (under uv python dir) is listed as a Global interpreter -- no "(uv)" label -- and is grouped/ranked like other standalone Pythons.
5. After opening the venv's parent as a workspace, confirm the uv virtual environment still shows as a uv environment.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:interpreter @:console @:modal

Why these tags?

Tag	Source
@:critical	Always runs (required)
@:interpreter	Changed files
@:console	Changed files
@:modal	Touched test file

More on automatic tags from changed files.

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