Clear junction properly when uninstalling Python versions on Windows

[woodruffw]

Summary

Reproduces and fixes #18793.

Previously, when uninstalling Python versions on Windows, we'd remove junctions (i.e. soft links) for the minor version after deleting the installation itself. This worked correctly on Linux and macOS but not on Windows, since on Windows we'd call junction::get_target (via PythonMinorVersionLink::exists), which would fail because the junction would be dangling following the deletion. Specifically, read_target returns None, short circuiting the target_directory check.

The fix here is to reorder the uninstallation flow so that we precompute and remove the links before the underlying installations are deleted. I've added two tests that both reproduced the behavior and now demonstrate the fix working.

Note: 81c27ba shows a smaller alternative fix -- instead of reordering the installation flow, we can change the "entry exists" logic on Windows to not require that the target still exists. I believe this would also be functionally correct, but I think reordering the uninstallation flow makes more sense (in terms of eliminating the surprising state rather than trying to work around it).

Test Plan

Look at me, I am the test plan now.

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks

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_{Comparing ww/repro-18793 (d1980c2) with main (6571827)}

[woodruffw]

Specifically, this is the problematic check in junction:

https://docs.rs/junction/1.4.2/src/junction/internals.rs.html#116-118

Path::exists() follows the junction and in this case the target has already been deleted, so junction::exists() fails-fast instead of proceeding to check the junction's actual metadata.