fix: Fix cancellation handling in channel processor test

The test ProcessWithChannel_WithCancellation_ShouldCancelGracefully was failing
because cancellation could occur before any items were written to the channel.

Changes:
- Added TaskCompletionSource to signal when processing starts
- Wait for processing to begin before triggering cancellation
- Reduced cancellation delay from 200ms to 50ms after processing starts

This ensures at least some items are processed before cancellation, making
the test behavior more predictable and reliable.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: thomhurst

EnumerableAsyncProcessor.UnitTests/ChannelBasedAsyncProcessorTests.cs

@@ -153,13 +153,20 @@ public async Task ProcessWithChannel_WithCancellation_ShouldCancelGracefully()
         var processedItems = new List<int>();
         var lockObj = new object();
         var items = Enumerable.Range(1, itemCount);
+        var processingStarted = new TaskCompletionSource<bool>();
 
         using var cts = new CancellationTokenSource();
         var options = ChannelProcessorOptions.CreateUnbounded(consumerCount: 2);
 
         // Act
         var processor = items.ForEachWithChannelAsync(async item =>
         {
+            // Signal that processing has started
+            if (item == 1)
+            {
+                processingStarted.TrySetResult(true);
+            }
+            
             // Don't pass the token to Task.Delay to allow some items to complete
             await Task.Delay(10);
 
@@ -173,10 +180,11 @@ public async Task ProcessWithChannel_WithCancellation_ShouldCancelGracefully()
             }
         }, options, cts.Token);
 
-        // Cancel after a short delay
+        // Wait for processing to start, then cancel after a short delay
         _ = Task.Run(async () =>
         {
-            await Task.Delay(200);
+            await processingStarted.Task;
+            await Task.Delay(50); // Give time for some items to be processed
             cts.Cancel();
         });