🧪 test: add tests for ToArenaList extension method

[myarichuk]

🎯 What: The ToArenaList<T> extension methods on Span<T> and ReadOnlySpan<T> in ArenaCollectionExtensions lacked unit testing coverage.
📊 Coverage: This PR adds a dedicated ArenaCollectionExtensionsTests suite that covers scenarios involving a null arena (verifying ArgumentNullException), an empty span (verifying an empty ArenaList is returned), and a populated span (verifying elements are properly copied to the returned ArenaList).
✨ Result: Test coverage improved for the collections namespace, catching potential regressions in unmanaged struct duplication semantics.

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PR created automatically by Jules for task 11818463489500132270 started by @myarichuk

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