CI - check for real if proto-derived files are up to date

[pavoljuhas]

• Rename check script build-changed-protos --> protos-up-to-date
• Excise unused code for getting base revision
• Exit with error status if dev_tools/build-protos.sh changes any content
• Fail the CI if generated protos are not up to date

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.83%. Comparing base (756abe3) to head (c3fd3dc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6747   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files        1077     1077           
  Lines       92558    92558           
=======================================
  Hits        90555    90555           
  Misses       2003     2003           

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[pavoljuhas]

Confirming that CI can now detect

• out-of-sync pb2.py files - https://github.com/quantumlib/Cirq/actions/runs/11063441886/job/30739564907
• orphaned pb2.py files - https://github.com/quantumlib/Cirq/actions/runs/11065003247/job/30743711857
• missing pb2.py files - https://github.com/quantumlib/Cirq/actions/runs/11065067820/job/30743875942

Ready for review.

[mhucka]

LGTM.