Add Immutable HashMap implementation

[koushik-sai]

Description

This pull request adds an immutable HashMap implementation using separate chaining.

Key Features

• Generic key and value support
• Immutable design: no modification of existing instances
• All update operations return a new ImmutableHashMap
• Thread-safe by design due to immutability

Implementation Details

• Uses separate chaining for collision handling
• Does not modify or extend existing Map or HashMap implementations
• Handles null keys safely
• Added with minimal impact to existing codebase

Notes

• Designed for educational and algorithmic clarity
• Fully compatible with strict compiler settings (-Werror)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.91%. Comparing base (48e02b3) to head (2c3a22d).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...tastructures/hashmap/hashing/ImmutableHashMap.java	92.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7183      +/-   ##
============================================
+ Coverage     78.90%   78.91%   +0.01%     
- Complexity     6858     6875      +17     
============================================
  Files           771      773       +2     
  Lines         22688    22722      +34     
  Branches       4457     4461       +4     
============================================
+ Hits          17901    17931      +30     
- Misses         4072     4074       +2     
- Partials        715      717       +2     

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