Respect global Python version pins in uv tool run and uv tool install

[zanieb]

Closes #12921

For uv tool run, we'll just use the global Python version for all invocations without an explicit alternative request (i.e., via the --python flag).

For uv tool install, it's a bit more complicated:

• If the tool is not installed, we'll use the global Python version
• If the tool is already installed, we won't change the Python version unless --reinstall or --python is used
• If the tool was installed with --python, we won't use the global Python version, unless the tool is uninstalled first

The behavior can be demonstrated as follows

$ uv python pin --global 3.12
$ uv tool install flask  # uses 3.12
$ uv tool install flask  # no-op
$ uv python pin --global 3.13
$ uv tool install flask  # no-op
$ uv tool install flask --reinstall  # uses 3.13
$ uv tool install flask -p 3.12 # uses 3.12
$ uv tool install flask  # no-op
$ uv tool install flask --reinstall # uses 3.12

This is a little more complicated than always reinstalling when the global Python version pin changes, but I think it's probably more intuitive when actually using the tool. We briefly touched on this when adding global version pins at #12115 (comment)

Minor note: I need to do a self-review of this implementation, as it's a little awkward to encode this behavior in the existing logic.

[zanieb]

I think this could be refactored to avoid the repeated else branches, but the logic should be correct. The comment needs more detail.

[zanieb]

@jtfmumm want to give this a look and see if we're aligned on the behavior?

[jtfmumm]

@jtfmumm want to give this a look and see if we're aligned on the behavior?

The behavior makes sense to me, and is consistent with how I was thinking about it before.