fix(venv): a ~ in venv.location hides every created virtualenv

[VXNCXNX]

A ~ in venv.location is expanded when a virtualenv is created but not when
one is looked up, so every pdm venv subcommand reports nothing after a
successful create.

With venv.location = ~/my-venvs:

before:
$ pdm venv create
Virtualenv /home/me/my-venvs/demo-DmFqMEeT-3.12 is created successfully
$ pdm venv list
Virtualenvs created with this project:

$ pdm venv purge --force
No virtualenvs to purge, quitting.
$ pdm venv activate 3.12
[PdmUsageError]: No virtualenv with key '3.12' is found, must be one of [].

after:
$ pdm venv list
-  3.12: /home/me/my-venvs/demo-dSE-z8Fz-3.12
$ pdm venv activate 3.12
source /home/me/my-venvs/demo-dSE-z8Fz-3.12/bin/activate

list, remove, activate, purge and pdm venv --path are all affected. The
venv exists and works, it is simply invisible to pdm.

Cause

Backend.get_location expands the path:

venv_parent = Path(self.project.config["venv.location"]).expanduser()

but the two iterators do not:

venv_parent = Path(project.config["venv.location"])

So the venv is created under /home/me/my-venvs and then looked for under a
directory literally named ~, which is also why a stray ~/ directory appears
in the working directory.

The fix

One get_venv_parent(project) helper doing the expansion, used by
get_location, iter_venvs and iter_central_venvs, so the three cannot drift
apart again.

Verification

test_venv_list_with_tilde_location sets a ~ location against a fake home,
creates a venv and checks pdm venv list finds it.

Removing the expansion from the helper fails it, and the failure names the
symptom exactly:

FAILED tests/cli/test_venv.py::test_venv_list_with_tilde_location - AssertionError: assert False
 +  where False = is_relative_to(PosixPath('.../fake-home') / 'my-venvs')
 +    where PosixPath('~/my-venvs/test_venv_list_with_tilde_loca0-xVyO888H-3.12').is_relative_to

tests/cli/test_venv.py is 36 passed. On the wider suite the baseline is 1346
passed on a stashed tree and 1347 with this change, no failures either way.

Two tests are excluded from that count because they fail identically on an
unmodified tree here: test_create_venv_in_project[venv-True] (this sandbox's
python -m venv --prompt exits non-zero) and the OIDC tests in
tests/cli/test_publish.py, which need pytest-httpx.

ruff check and ruff format --check are clean.

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Disclosure: written with AI assistance (Claude Code). I produced the before and after by running the real pdm venv commands against a ~ location, and ran the mutation check and the suite baseline myself.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.37%. Comparing base (5a6ea88) to head (627774a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3847   +/-   ##
=======================================
  Coverage   88.36%   88.37%           
=======================================
  Files         121      121           
  Lines       13249    13251    +2     
  Branches     2251     2251           
=======================================
+ Hits        11708    11710    +2     
  Misses        969      969           
  Partials      572      572           

Flag	Coverage Δ
unittests	88.25% <100.00%> (+<0.01%)	⬆️

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