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

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.88%. Comparing base (31bf130) to head (5328d9a).

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     74.86%   74.88%   +0.02%     
- Complexity     5482     5490       +8     
============================================
  Files           682      683       +1     
  Lines         19012    19027      +15     
  Branches       3670     3675       +5     
============================================
+ Hits          14233    14249      +16     
+ Misses         4224     4223       -1     
  Partials        555      555              

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[DenizAltunkapan]

please add here another detailed explanation above the class. For example by providing a link and most importantly adding the time- and space complexity :)

[Nishitha0730]

okay:)

[DenizAltunkapan]

could you please add testcases for 1x1 matrices and not-rectangular matrices?

[Nishitha0730]

Sure:)