Refactor Caesar Algorithm Implementation

[StPfeffer]

Description

Refactored the Caesar algorithm to handle larger inputs and rotations while preserving the existing code structure. Included additional tests for improved coverage.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I ran bellow commands using the latest version of rust nightly.
• [x] I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• [x] I ran cargo fmt just before my last commit.
• [x] I ran cargo test just before my last commit and all tests passed.
• [x] I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• [x] I added my algorithm to DIRECTORY.md with the correct link.
• [x] I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.79%. Comparing base (1020ea2) to head (44bdb8b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #717 +/- ## ========================================== + Coverage 94.77% 94.79% +0.02% ========================================== Files 298 298 Lines 22194 22261 +67 ========================================== + Hits 21035 21103 +68 + Misses 1159 1158 -1 ```

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