Fix: Convert readonly to const in C++ transformations

[konard]

Summary

This PR fixes the transformation of C# readonly fields to properly use const in C++ output, addressing issue #56.

Changes Made

• Updated the regex pattern on line 127 in CSharpToCppTransformer.cs to include const in the replacement string for string types
• The pattern (const|static readonly) string now correctly outputs inline static const std::string instead of inline static std::string
• Line 130 already handled non-string types correctly with const output

Before

public: inline static std::string ExceptionContentsSeparator = "---";

After

public: inline static const std::string ExceptionContentsSeparator = "---";

Testing

• All existing unit tests pass
• Added test cases in experiments/ folder to verify the transformation works correctly
• Verified that both readonly and const C# keywords are now properly converted to const in C++

Test Plan

• Run existing unit tests (dotnet test)
• Create and run transformation tests with various readonly field types
• Verify string types now output with const
• Verify non-string types continue to work correctly
• Confirm no regressions in existing transformations

🤖 Generated with Claude Code

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Resolves #56