Exclude proto generated files in coverage test

[AzulRadio]

Signed-off-by: youhy haoyuan2019@outlook.com

Increase the test coverage rate for gz-msgs5 from ~18% to ~85%.

• Part of https://github.com/gazebosim/gz-sim/issues/1575

Summary

All codes generated by proto all follow this pattern: *.pb.cc or *.pb.h

To test this, follow https://gazebosim.org/docs/all/contributing#test-coverage to generate coverage report for gz-msgs5

Before:

After:

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.