Fix: Out-of-bounds issue in counting_sort_string.cpp (#3055)

[The-AarushiSingh]

Description of Change

Fixed an out-of-bounds access issue in sorting/counting_sort_string.cpp by pre-allocating the output string with the correct size.

The previous implementation initialized an empty string and wrote to specific indices, causing undefined behavior and potential crashes.
Now, the output string is initialized as string output(n, ' '); to ensure safe memory access and correct sorting behavior.

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Checklist

• Added description of change
• File name follows File name guidelines
• Tested with sample inputs to verify correct output
• Added inline documentation for clarity
• PR title follows semantic commit guidelines
• I acknowledge that all my contributions will be made under the project's license

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Notes

This PR addresses issue #3055 and ensures countSort runs safely without segmentation faults or memory corruption.