Add polyfill for Path.Join(...)

[Tyrrrz]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a polyfill for the Path.Join(params string?[]) method introduced in .NET Core 3.0, enabling its use on older framework versions. The implementation refactors the existing EndsInDirectorySeparator method to use a new helper method IsDirectorySeparator, which is then reused in the Join implementation.

• Introduces a private IsDirectorySeparator(char) helper method for cleaner code reuse
• Refactors EndsInDirectorySeparator to use expression-bodied syntax with the new helper
• Implements Path.Join(params string?[]) that concatenates paths with proper separator handling
• Adds test coverage for the new Join method

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
PolyShim/NetCore30/Path.cs	Adds IsDirectorySeparator helper, refactors EndsInDirectorySeparator, and implements Path.Join polyfill with appropriate preprocessor directives for different framework versions
PolyShim.Tests/NetCore30/PathTests.cs	Adds Join_Test with three test cases covering path concatenation scenarios including trailing/leading separators and rooted paths

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (84385aa) to head (8e82480).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@     Coverage Diff      @@
##   master   #44   +/-   ##
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