Fix SSE fallback test race condition

- Publish event in background task after GetStreamAsync starts
- Prevents timeout by ensuring event is published after endpoint subscribes
- Fixes TaskCanceledException after 200s timeout in CI

Author: ANcpLua

SWEN3.Paperless.RabbitMq.Tests/Unit/SseExtensionsFallbackTests.cs

@@ -29,17 +29,15 @@ public async Task MapSse_OnNet8_ShouldWriteCorrectSseFormat()
         var sseStream = server.Host.Services.GetRequiredService<ISseStream<Messages.SseTestEvent>>();
 
         // Act
-        // Start the request
+        // Start the SSE request (will block until data is available)
         var responseTask = client.GetStreamAsync("/sse", TestContext.Current.CancellationToken);
 
-        // Give it a moment to connect
-        await Task.Delay(50, TestContext.Current.CancellationToken);
-
-        // Publish an event
-        sseStream.Publish(new Messages.SseTestEvent { Id = 1, Message = "Hello" });
-
-        // Allow processing
-        await Task.Delay(50, TestContext.Current.CancellationToken);
+        // Start a task to publish the event after giving the endpoint time to subscribe
+        _ = Task.Run(async () =>
+        {
+            await Task.Delay(100, TestContext.Current.CancellationToken);
+            sseStream.Publish(new Messages.SseTestEvent { Id = 1, Message = "Hello" });
+        }, TestContext.Current.CancellationToken);
 
         // Assert
         var stream = await responseTask;