Await chained key computation in AsyncEnumerable ordering

[Copilot]

ThenBy and ThenByDescending discard chained key-computation tasks, allowing sorting to read uninitialized keys when selectors suspend and silently ignoring selector exceptions.

• Fix: Await each subsequent sorter’s key computation before sorting.
• Regression coverage: Add deterministic suspended-selector cases for secondary and third-level ordering in both directions, plus exception propagation when primary keys differ.

if (_next is not null)
{
    await _next.ComputeKeysAsync(elements, count, cancellationToken);
}

• Fixes AsyncEnumerable.ThenBy with async key selector throws NullReferenceException: secondary keys not awaited #135343

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[eiriktsarpalis]

/ba-g test failure is unrelated.