feat: Add ValidParentheses utility with tests

[Ruturaj-007]

Summary

This PR adds a new utility class ValidParentheses that checks if a given string of parentheses is valid.
It supports '()', '{}', and '[]' and ensures correct nesting and ordering.

Features

• isValid(String str) checks if the input string has valid parentheses.
• Comprehensive JUnit tests included in ValidParenthesesTest.
• Covers both valid and invalid cases, including empty strings.

Testing

All tests have been verified locally and passed successfully.

References

• Problem description: LeetCode: Valid Parentheses
• Example usage included in the main method.

This contribution follows the repository’s naming conventions and code style.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.80%. Comparing base (f0fb971) to head (de37275).

Files with missing lines	Patch %	Lines
...ava/com/thealgorithms/stacks/ValidParentheses.java	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6734      +/-   ##
============================================
- Coverage     76.80%   76.80%   -0.01%     
- Complexity     6047     6054       +7     
============================================
  Files           719      720       +1     
  Lines         20342    20358      +16     
  Branches       3940     3945       +5     
============================================
+ Hits          15624    15635      +11     
- Misses         4127     4130       +3     
- Partials        591      593       +2     

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[Ruturaj-007]

Hi @DenizAltunkapan, @yanglbme, and @alxkm

All linter issues (Checkstyle, PMD, Clang format) have been fixed.
ValidParentheses now uses Deque and all if statements have braces to satisfy Checkstyle.
JUnit tests cover valid, invalid, and null inputs.
Patch coverage is currently 81.25%; additional edge-case tests can be added if needed.

Looking forward to your review!

[alxkm]

Looks like some similar class already exist
https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/ValidParentheses.java

[Ruturaj-007]

Thanks for pointing that out, @alxkm!
I noticed there’s already a ValidParentheses class under com.thealgorithms.strings, but my implementation is placed under com.thealgorithms.stacks, emphasizing the stack-based approach to solving the problem.

This version also enforces the utility class pattern, satisfies PMD and Checkstyle rules, and focuses on algorithmic demonstration rather than input validation.

If preferred, you may merge these improvements into the existing one or move this implementation to a more suitable location.

I'm grateful for this experience!