[Context]: add ProcessStream for IAsyncEnumerable

[rian-be]

Summary

Add ProcessStream — support for IAsyncEnumerable sources so items are processed as they arrive.

Goal

Let context-scoped work consume and process async streams item-by-item as data flows in, rather than after materializing a full collection.

Problem

Current batch primitives take IEnumerable<T>. For unbounded/live sources (queue, paginated endpoints), callers must materialize entire collections first, hurting latency and memory.

Example

await foreach (var item in manager.ProcessStream(ctx, GetItemsAsync()))
{
    await Process(item);
}

Considerations

• IAsyncEnumerable processing stream.
• Ordering implications under streaming (pair with Ordered/Unordered).
• Cancellation propagates through the stream.
• Per-item context isolation preserved.

Non-Goals

• Not bounded by concurrency here unless composed with Map/MaxConcurrency.