Convert blocking Task.WaitAll to async/await in concurrency tests

[Copilot]

xunit analyzer warns that test methods using blocking task operations (Task.WaitAll) can cause deadlocks.

Changes

• Converted two test methods to async Task signatures
• Replaced Task.WaitAll(tasks.ToArray()) with await Task.WhenAll(tasks)

Before:

[Fact]
public void Concurrent_Inserts_Are_Thread_Safe()
{
    // ...
    Task.WaitAll(tasks.ToArray());
}

After:

[Fact]
public async Task Concurrent_Inserts_Are_Thread_Safe()
{
    // ...
    await Task.WhenAll(tasks);
}

Original prompt

fix this xunit warning: Test methods should not use blocking task operations, as they can cause deadlocks. Use an async test method and await instead.

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