feat: add contains() method to DynamicArray

[Neha-2005-VCE]

Added contains(E element) method to DynamicArray.

• Uses Objects.equals for null-safe comparison
• Iterates only over active elements (size)
• Added unit tests including null case

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.33%. Comparing base (504b528) to head (100441e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7270      +/-   ##
============================================
+ Coverage     79.26%   79.33%   +0.06%     
- Complexity     6984     6994      +10     
============================================
  Files           781      781              
  Lines         22958    22962       +4     
  Branches       4510     4511       +1     
============================================
+ Hits          18197    18216      +19     
+ Misses         4034     4020      -14     
+ Partials        727      726       -1     

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