Bump ProtoBuf dependency to ~1.2.1 (supports client stub codegen)

[csvance]

Also add tests for codegen.

With any luck we can tag this as 1.0.0 when its merged.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.53%. Comparing base (ba8b793) to head (7e30805).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   83.90%   90.53%   +6.62%     
==========================================
  Files           7        7              
  Lines         528      528              
==========================================
+ Hits          443      478      +35     
+ Misses         85       50      -35     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.