Fix: resolve unsigned loop bug in trapped rainwater implementation

[Akash-Karma]

Description of Change

Checklist

• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• 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: Fixed a bug caused by reverse iteration using std::size_t, which could lead to underflow.

Changes:

• Fixed reverse loop condition
• Improved type safety using uint64_t
• All test cases pass