perf: defer Python installer import

[yangfan-yf-yf]

Fixes #3673.

Summary

• move the PythonVersion import behind TYPE_CHECKING, so command registration does not load pbs_installer
• extend the clean-subprocess startup regression to cover the Python installer package
• update the existing Improve the startup performance #3673 news fragment

Why

pdm.project.core only uses PythonVersion in deferred annotations. Importing it at module load time also imports the Python installer stack for commands that never install a Python interpreter.

Validation

• the startup regression failed before the change with pbs_installer loaded and passes after the change
• targeted startup and minimum/maximum matching tests: 3 passed
• tests/cli/test_venv.py and tests/test_project.py: 104 passed, 3 skipped
• non-integration suite: 1279 passed, 25 skipped, 1 xfailed; the remaining 4 Windows environment failures were reproduced on unchanged main (symlink privilege and unavailable POSIX cat/echo commands)
• Ruff check and format check passed for the changed Python files
• Codespell and git diff --check passed
• Mypy passed for src/pdm/project/core.py; the full locally resolved run had the same 38 dependency-version errors on this branch and unchanged main

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.16%. Comparing base (b3cb02e) to head (d9a9a04).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3836      +/-   ##
==========================================
- Coverage   88.16%   88.16%   -0.01%     
==========================================
  Files         121      121              
  Lines       13221    13220       -1     
  Branches     2246     2246              
==========================================
- Hits        11656    11655       -1     
  Misses        982      982              
  Partials      583      583              

Flag	Coverage Δ
unittests	88.04% <ø> (-0.01%)	⬇️

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