Add ReservoirSampling algorithm to randomized module

[cureprotocols]

Algorithm Overview:

• Efficient for selecting k random elements from a stream of unknown size
• Commonly used in streaming systems, big data pipelines, and memory-limited environments
• Time Complexity: O(n)
• Space Complexity: O(k)

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Implementation Details:

• Class: ReservoirSampling
• Package: com.thealgorithms.randomized
• JavaDoc included for class and method
• Demonstration included in main() method
• File name and class name follow PascalCase
• Fully formatted using clang-format

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Reference:

• Reservoir Sampling - Wikipedia

Author: Michael Alexander Montoya (@cureprotocols)

• 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

[DenizAltunkapan]

What if sampleSize > stream.length? Perhaps there should be error handling for invalid input, the rest lgtm

[cureprotocols]

✅ Added input validation for sampleSize > stream.length as requested — thanks for the suggestion, @DenizAltunkapan!

Let me know if anything else is needed — happy to improve further 🙌