Add matrix multiplication with double[][] and unit tests

[Nishitha0730]

Summary

This PR adds an implementation of standard matrix multiplication using double[][] arrays in the com.thealgorithms.matrix package.

• The MatrixMultiplication class multiplies two 2D matrices with floating-point support.
• Includes input validation for null, empty, and incompatible dimensions.

Tests

Added a comprehensive JUnit 5 test class MatrixMultiplicationTest with:

• Valid multiplication tests
• Edge cases: null input, empty matrix, incompatible dimensions
• Floating-point accuracy check using EPSILON

Please let me know if any changes are needed!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.77%. Comparing base (2722b0e) to head (8c1beaa).
⚠️ Report is 337 commits behind head on master.

Files with missing lines	Patch %	Lines
...com/thealgorithms/matrix/MatrixMultiplication.java	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6417   +/-   ##
=========================================
  Coverage     74.76%   74.77%           
- Complexity     5477     5483    +6     
=========================================
  Files           682      683    +1     
  Lines         19012    19027   +15     
  Branches       3670     3675    +5     
=========================================
+ Hits          14215    14228   +13     
  Misses         4245     4245           
- Partials        552      554    +2     

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