feat: add optimized Digit DP template and unit tests

[premmsharma122]

Types of Changes

• New algorithm implementation

Description

This PR introduces a clean and optimized template implementation for the Digit Dynamic Programming (Digit DP) technique under the dynamicprogramming package.

Key enhancements integrated into this template include:

1. Magic Numbers Elimination:
   Replaced hardcoded dimensions with a clear MAX_DIGIT_SUM constant representing the maximum theoretical digit sum for a 64-bit signed long integer.

2. Explicit Complexity Documentation:
   Added dedicated JavaDoc comments documenting the state-space complexity:

   • Time Complexity: O(number_of_digits × target_sum × 10)
   • Space Complexity: O(number_of_digits × target_sum × 2)

3. Educational Enhancements:
   Included detailed inline documentation, state-transition explanations, pruning optimization comments, and practical JavaDoc examples for easier understanding of Digit DP concepts.

4. Comprehensive Test Coverage:
   Added multiple JUnit test cases covering:

   • standard ranges,
   • zero-bound handling,
   • invalid ranges,
   • impossible target sums,
   • and edge-case validations.

Related Issue

Closes #7429

Checklist

• My code follows the code style of this project (mvn checkstyle:check passed locally).
• I have performed a self-review of my own code.
• I have added comprehensive test coverage that passes successfully (mvn test passed locally).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.65%. Comparing base (4b8099c) to head (abc31a4).

Files with missing lines	Patch %	Lines
.../com/thealgorithms/dynamicprogramming/DigitDP.java	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7430      +/-   ##
============================================
+ Coverage     79.63%   79.65%   +0.01%     
- Complexity     7241     7254      +13     
============================================
  Files           800      801       +1     
  Lines         23608    23636      +28     
  Branches       4646     4654       +8     
============================================
+ Hits          18800    18827      +27     
+ Misses         4055     4054       -1     
- Partials        753      755       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.