Add comprehensive test suite for Program.Code() encoding/decoding method

[Copilot]

The Program.Code() method implements a bidirectional Caesar cipher with pseudo-random key-based encoding but lacked test coverage.

Changes

• Created JoinFS.Tests project with xUnit framework targeting .NET 8.0

  • Platform-independent (net8.0 vs net8.0-windows) to support cross-platform CI/CD
  • Added standalone copy of Program.Code() to avoid Windows Forms dependencies

• Implemented 34 test cases covering:

  • Round-trip encoding/decoding verification
  • Key sensitivity and deterministic behavior
  • Edge cases (null, empty, invalid characters, boundary conditions)
  • Real-world scenarios (URLs, file paths, long strings)
  • Character range validation (printable ASCII '!' to '~')

• Repository configuration

  • Added test project to solution
  • Updated .gitignore for test artifacts

Example Test

[Theory]
[InlineData("Hello World")]
[InlineData("https://example.com")]
[InlineData("Path/To/File.txt")]
public void Code_RoundTrip_RestoresOriginalString(string original)
{
    string encoded = ProgramCode.Code(original, true, TestKey);
    string decoded = ProgramCode.Code(encoded, false, TestKey);
    
    Assert.NotEqual(original, encoded);
    Assert.Equal(original, decoded);
}

All 34 tests pass (Duration: ~55ms).

Original prompt

Write tests for Program.Code(...).

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[tuduce]

Lgtm