Fix codecov.yml: move after_n_builds to codecov.notify

[blink1073]

Summary

• after_n_builds is not a valid field under coverage.status.project or coverage.status.patch
• Moved it to codecov.notify.after_n_builds per the Codecov schema
• Validated with curl --data-binary @codecov.yml https://codecov.io/validate

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.48%. Comparing base (705a633) to head (9c4d9fe).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (91.48%) is below the target coverage (95.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
+ Coverage   90.07%   91.48%   +1.41%     
==========================================
  Files           6        6              
  Lines         423      423              
==========================================
+ Hits          381      387       +6     
+ Misses         42       36       -6     

Flag	Coverage Δ
linux-py3.10	90.07% <ø> (?)
linux-py3.11	90.07% <ø> (?)
linux-py3.12	90.07% <ø> (ø)
linux-py3.13	90.07% <ø> (?)
linux-py3.14	90.07% <ø> (?)
linux-py3.14t	90.07% <ø> (?)
macos-latest	90.07% <ø> (?)
ubuntu-flatpak	90.78% <ø> (?)
ubuntu-snap	89.83% <ø> (?)
windows-latest	90.30% <ø> (?)

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see 1 file with indirect coverage changes

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