test: improve coverage for Strings utility class

[RoiSoleil]

J'ai amélioré la couverture de tests pour la classe utilitaire Strings dans le module org.moreunit.core.test.

• Ajout de tests pour les méthodes join utilisant StringBuilder (tableaux et collections).
• Ajout de tests pour les cas aux limites (chaînes vides ou nulles) de countOccurrences.
• Ces ajouts n'altèrent en rien le comportement de production et évitent d'introduire des tests instables liés à l'interface utilisateur.

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PR created automatically by Jules for task 18221104773447823059 started by @RoiSoleil

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.15%. Comparing base (229eb6b) to head (ff4b6fd).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #282      +/-   ##
============================================
+ Coverage     75.11%   75.15%   +0.03%     
- Complexity     3312     3317       +5     
============================================
  Files           422      422              
  Lines         14561    14575      +14     
  Branches       1271     1271              
============================================
+ Hits          10938    10954      +16     
+ Misses         3090     3089       -1     
+ Partials        533      532       -1     

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