Audit and ensure all branch rules backend error messages are properly translated

Summary

In !211128 (comment 2863406253), we discovered that some backend validation error messages may not be properly externalized for translation. We've already addressed protected_branch.rb and squash_option.rb, but there may be other files in the branch rules domain that need similar treatment.

Scope

Audit all backend files related to branch rules functionality to ensure error messages, validation messages, and user-facing strings are properly externalized for translation.

References

https://docs.gitlab.com/development/i18n/externalization/#ruby-files