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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[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.