test: enhance GenericHashMapUsingArrayTest with comprehensive edge case coverage

[lmj798]

Summary

Add comprehensive test coverage for GenericHashMapUsingArray to improve code quality and edge case handling.

Why These Tests Matter

1. Null & Boundary Handling

• Null key validation: Prevents null pointer exceptions that would crash the application
• Null value storage: Ensures the Map correctly handles null values (note: containsKey returns false for null values—this is an implementation limitation)
• Empty string keys: Often overlooked but commonly used as placeholders in real-world scenarios

2. Numeric Edge Cases

• Negative integer keys: Hash calculation behaves differently for negative numbers in implementation
• Zero as key: Boundary case where hash value equals zero
• Load factor rehash trigger: Validates the expansion mechanism works correctly—this is a critical performance path

3. Hash Collision Handling

• Same bucket collisions: Integer(1) and Integer(17) hash to the same bucket, verifying链表 traversal logic works correctly
• Update in collision bucket: Ensures updating a value in a colliding bucket doesn't affect other elements

4. String Stress Testing

• Special characters: Real-world keys may contain !@#$%^&*() and similar characters
• Long strings (1000 chars): Tests memory allocation and hash calculation under stress
• Parameterized tests: Validates consistent behavior across varying key lengths

Test Results

Metric	Before	After	Improvement
Instructions	98%	99%	+1%
Branches	86%	93%	+7%
Test Count	8	24	+16
All tests pass

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.44%. Comparing base (9aa2544) to head (5167f86).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7300      +/-   ##
============================================
+ Coverage     79.42%   79.44%   +0.01%     
- Complexity     7062     7065       +3     
============================================
  Files           788      788              
  Lines         23129    23129              
  Branches       4548     4548              
============================================
+ Hits          18371    18375       +4     
+ Misses         4024     4022       -2     
+ Partials        734      732       -2     

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