feat: emit post_lock after writing pyproject.toml and pdm.lock in add…

[jantrienes]

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.

New lockfile is written before post_lock is emitted. Implementation analogous to #1225
Fixes #3285.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.32%. Comparing base (3110fce) to head (096356a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3291      +/-   ##
==========================================
- Coverage   85.33%   85.32%   -0.01%     
==========================================
  Files         112      112              
  Lines       11402    11402              
  Branches     2487     2487              
==========================================
- Hits         9730     9729       -1     
- Misses       1143     1144       +1     
  Partials      529      529              

Flag	Coverage Δ
unittests	85.12% <100.00%> (-0.01%)	⬇️

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