fix: write project TOML files atomically

[lodyai]

Pull Request Checklist

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

Describe what you have changed in this PR.

• Write project TOML content through atomic_open_for_write so interrupted writes do not truncate pyproject.toml or lockfiles.
• Preserve the line separator detected by tomlkit and disable implicit newline translation while writing the temporary file.
• Cover pyproject.toml, pdm.lock, pylock.toml, and CRLF preservation with regression tests.

Fixes #3870.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.79%. Comparing base (b6acd0e) to head (81297cb).

Files with missing lines	Patch %	Lines
src/pdm/project/toml_file.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3871      +/-   ##
==========================================
- Coverage   88.79%   88.79%   -0.01%     
==========================================
  Files         120      120              
  Lines       13322    13330       +8     
  Branches     2265     2267       +2     
==========================================
+ Hits        11829    11836       +7     
  Misses        931      931              
- Partials      562      563       +1     

Flag	Coverage Δ
unittests	88.67% <90.90%> (-0.01%)	⬇️

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