Disallow "stringable_for_to_string" CS rule

[phansys]

The error related to the call-site variance introduced at #3014 will be attended in a different PR.

See PHP-CS-Fixer/PHP-CS-Fixer#9248.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.75%. Comparing base (e60de54) to head (4313a89).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3012   +/-   ##
=======================================
  Coverage   78.75%   78.75%           
=======================================
  Files         169      169           
  Lines        8695     8695           
=======================================
  Hits         6848     6848           
  Misses       1847     1847           

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