style: include SPP_TOSTRING_ON_STRING

[vil02]

Continuation of #5126.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.62%. Comparing base (295e743) to head (7f73bcb).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5183      +/-   ##
============================================
+ Coverage     38.60%   38.62%   +0.01%     
- Complexity     2377     2379       +2     
============================================
  Files           516      516              
  Lines         15385    15385              
  Branches       2957     2957              
============================================
+ Hits           5939     5942       +3     
+ Misses         9159     9158       -1     
+ Partials        287      285       -2     

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