ci: gate build workflow on unit checks

[cofin]

Summary

• add a cheap gate job to Test Build Configuration that waits for quality and all test-unit checks on the PR head SHA
• block source/wheel package build jobs until that gate passes
• enable fail-fast on the expensive MyPyC build and wheel-test matrices

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.78%. Comparing base (b8ad173) to head (b67744a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #470      +/-   ##
==========================================
- Coverage   70.78%   70.78%   -0.01%     
==========================================
  Files         438      438              
  Lines       52541    52541              
  Branches     7370     7370              
==========================================
- Hits        37191    37189       -2     
- Misses      12681    12682       +1     
- Partials     2669     2670       +1     

Flag	Coverage Δ
integration	56.59% <ø> (-0.01%)	⬇️
py3.10	67.85% <ø> (ø)
py3.11	67.87% <ø> (ø)
py3.12	67.87% <ø> (-0.01%)	⬇️
py3.13	67.87% <ø> (-0.01%)	⬇️
py3.14	69.95% <ø> (-0.01%)	⬇️
unit	57.45% <ø> (-0.01%)	⬇️

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