test: add happy path test for ArenaList.AsReadOnlySpan()

[myarichuk]

🎯 What: The testing gap addressed for ArenaList<T>.AsReadOnlySpan().
📊 Coverage: A happy path test was added that initializes an ArenaList<int>, adds a few items, calls AsReadOnlySpan(), and verifies both the length and contents.
✨ Result: Increased test coverage and confidence that the AsReadOnlySpan method properly creates a correct read-only view over the elements in the arena list.

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PR created automatically by Jules for task 6542578739033788653 started by @myarichuk

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