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

[myarichuk]

🎯 What: The testing gap addressed
The ArenaList<T>.AsSpan() method lacked a happy path test confirming its core functional requirement: providing a writable span that genuinely references and allows modification of the list's contents.

📊 Coverage: What scenarios are now tested
A test has been added that covers the full happy path logic:

• A new ArenaList is allocated and populated with values.
• AsSpan() returns a span whose properties (Length and elements) match the expected data correctly.
• Updating an element via the returned span correctly mutates the underlying ArenaList memory.

✨ Result: The improvement in test coverage
We now have reliable test coverage verifying that AsSpan() produces spans properly bound to the existing unmanaged data, ensuring safety and correctness when performing bulk or sliced array/span operations directly on the collection data.

---

PR created automatically by Jules for task 9328814400890749979 started by @myarichuk

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.