Implemented Complex Number Multiplication in the String folder

[muskansngh07]

Description:

This PR introduces a utility class ComplexNumberMultiplication in Java that multiplies two complex numbers represented as strings.

Features:

• Accepts two complex numbers in the format "a+bi" and "c+di".
• Returns the product as a string in the same format "x+yi".
• Uses the formula: (a + bi) × (c + di) = (ac - bd) + (ad + bc)i.
• Handles positive and negative integer parts.
• Includes a main method.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.80%. Comparing base (b312567) to head (717d8f8).

Files with missing lines	Patch %	Lines
...lgorithms/strings/ComplexNumberMultiplication.java	0.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6805      +/-   ##
============================================
- Coverage     77.84%   77.80%   -0.05%     
  Complexity     6394     6394              
============================================
  Files           733      734       +1     
  Lines         21333    21342       +9     
  Branches       4183     4183              
============================================
- Hits          16607    16605       -2     
- Misses         4058     4068      +10     
- Partials        668      669       +1     

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[muskansngh07]

hii @DenizAltunkapan @alxkm @yanglbme
if you could please review this?

[DenizAltunkapan]

@muskansngh07 Please always add the corresponding test class