🧪 test: add coverage for ArenaUtf8String.DecodeTo

[myarichuk]

🎯 What: The ArenaUtf8String.DecodeTo method was lacking unit tests, leaving its decoding logic uncovered.
📊 Coverage: Added tests to cover successful decoding to a char[] buffer (acting as Span<char>) for both standard ASCII strings and multibyte UTF-8 strings (containing emojis).
✨ Result: Enhanced test coverage and guaranteed that UTF-8 string decoding to characters behaves as expected without truncation or corruption.

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PR created automatically by Jules for task 11485840963802933074 started by @myarichuk

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