fix: correct FCFS template handling and improve self-tests

[harshitrajput2802]

Description of Change

Corrected the FCFS scheduling implementation by fixing a template type mismatch in the reference implementation used for verification. The previous implementation hardcoded the input type to uint32_t, preventing the helper function from correctly supporting arbitrary template parameters.

This update also:

• Fixed the template signature of the reference implementation to properly use the class template parameters.
• Replaced repeated std::srand(std::time(nullptr)) calls with a single std::mt19937 engine and uniform distributions for reliable randomized testing.
• Added namespace organization under scheduling::fcfs.
• Improved const-correctness and documentation with Doxygen comments.
• Preserved the existing FCFS scheduling behavior while strengthening correctness and maintainability.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Fixes the FCFS reference implementation to correctly support template parameters and improves randomized self-tests using a modern C++ random number generator.