Fix SSE fallback test race condition with ResponseHeadersRead

Changed the test to use HttpCompletionOption.ResponseHeadersRead which returns
as soon as the HTTP connection is established (headers received), then publishes
the event. This eliminates the race condition where the event could be published
before the endpoint subscribes to the SSE stream.

Previously, the test used Task.Delay(100ms) hoping the subscription would happen
in time, which caused timeouts in CI environments (GitHub Actions).

Added 5-second timeout for safety.

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

Co-Authored-By: Claude <[email protected]>

Author: ANcpLua

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

@@ -29,28 +29,24 @@ public async Task MapSse_OnNet8_ShouldWriteCorrectSseFormat()
         var sseStream = server.Host.Services.GetRequiredService<ISseStream<Messages.SseTestEvent>>();
 
         // Act
-        // Start the SSE request (will block until data is available)
-        var responseTask = client.GetStreamAsync("/sse", 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;
+        // Start the request
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        var request = new HttpRequestMessage(HttpMethod.Get, "/sse");
+        
+        // Use ResponseHeadersRead to return as soon as headers are received, keeping the stream open
+        using var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cts.Token);
+        response.EnsureSuccessStatusCode();
+        
+        var stream = await response.Content.ReadAsStreamAsync(cts.Token);
         using var reader = new StreamReader(stream, Encoding.UTF8);
-
-        // Read the stream. We expect:
-        // event: test-event\n
-        // data: {"id":1,"msg":"Hello"}\n
-        // \n
-
-        var line1 = await reader.ReadLineAsync(TestContext.Current.CancellationToken);
-        var line2 = await reader.ReadLineAsync(TestContext.Current.CancellationToken);
-        var line3 = await reader.ReadLineAsync(TestContext.Current.CancellationToken);
+        
+        // Publish an event *after* connection is established (headers received)
+        sseStream.Publish(new Messages.SseTestEvent { Id = 1, Message = "Hello" });
+        
+        // Read the stream
+        var line1 = await reader.ReadLineAsync(cts.Token);
+        var line2 = await reader.ReadLineAsync(cts.Token);
+        var line3 = await reader.ReadLineAsync(cts.Token);
 
         line1.Should().Be("event: test-event");
         line2.Should().Be("data: {\"id\":1,\"msg\":\"Hello\"}");