feat: infix to postfix conversion and evaluation with basic tests

[kaloa2025]

Summary

This pull request adds a complete implementation of Infix to Postfix Conversion along with Postfix Expression Evaluation to the Algorithms.Stack namespace.

Included in this PR:

• Conversion from infix expression → postfix expression using a stack
• Evaluation of postfix expressions with support for operators: +, -, *, /, ^
• Proper validation for:
  • Invalid characters
  • Mismatched parentheses
  • Insufficient operands
  • Division by zero
• XML documentation for clarity and educational value
• NUnit test cases covering valid expressions, invalid expressions, operator precedence, and edge cases

This algorithm is a classic application of stacks and fits perfectly under the existing Algorithms.Stack category in this repository.

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Checklist

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

❌ Patch coverage is 86.61417% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.82%. Comparing base (52e4ef3) to head (2ebe57f).

Files with missing lines	Patch %	Lines
Algorithms/Stack/InfixToPostfix.cs	86.61%	11 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
- Coverage   96.93%   96.82%   -0.11%     
==========================================
  Files         297      298       +1     
  Lines       12287    12414     +127     
  Branches     1781     1807      +26     
==========================================
+ Hits        11910    12020     +110     
- Misses        239      250      +11     
- Partials      138      144       +6     

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[siriak]

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