Improve SSE header handling and rename fallback test

Production changes:
- Replace Body.FlushAsync() with StartAsync() in SSE fallback implementation
- StartAsync() is more semantically correct for sending headers immediately
- Ensures headers reach client before awaiting first event

Test improvements:
- Rename test to MapSse_Fallback_ShouldWriteCorrectSseFormat (clearer intent)
- Remove Skip attribute - test is now reliable with continuous publishing
- Increase timeout to 30s for slower CI environments
- Set client timeout to infinite (test has its own 30s cancellation)

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

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

Author: ANcpLua

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

@@ -3,8 +3,8 @@ namespace SWEN3.Paperless.RabbitMq.Tests.Unit;
 public class SseExtensionsFallbackTests
 {
 #if !NET10_0_OR_GREATER
-    [Fact(Skip = "Flaky on CI runners; SSE fallback covered by integration flows")]
-    public async Task MapSse_OnNet8_ShouldWriteCorrectSseFormat()
+    [Fact]
+    public async Task MapSse_Fallback_ShouldWriteCorrectSseFormat()
     {
         // Arrange
         var hostBuilder = new WebHostBuilder()
@@ -26,12 +26,12 @@ public async Task MapSse_OnNet8_ShouldWriteCorrectSseFormat()
 
         using var server = new TestServer(hostBuilder);
         var client = server.CreateClient();
-        client.Timeout = TimeSpan.FromSeconds(10);
+        client.Timeout = Timeout.InfiniteTimeSpan;
         var sseStream = server.Host.Services.GetRequiredService<ISseStream<Messages.SseTestEvent>>();
 
         // Act
         using var cts = CancellationTokenSource.CreateLinkedTokenSource(TestContext.Current.CancellationToken);
-        cts.CancelAfter(TimeSpan.FromSeconds(10));
+        cts.CancelAfter(TimeSpan.FromSeconds(30));
 
         var responseTask = client.GetAsync("/sse", HttpCompletionOption.ResponseHeadersRead, cts.Token);
 

SWEN3.Paperless.RabbitMq/Sse/SseExtensions.cs

@@ -115,8 +115,8 @@ async IAsyncEnumerable<SseItem<object>> StreamEventsAsync([EnumeratorCancellatio
                 context.Response.Headers.Connection = "keep-alive";
                 context.Response.Headers.ContentType = "text/event-stream";
 
-                // Flush headers immediately so clients receive them before data arrives
-                await context.Response.Body.FlushAsync(ct).ConfigureAwait(false);
+                // Start the response so headers are sent immediately, even before the first event
+                await context.Response.StartAsync(ct).ConfigureAwait(false);
 
                 await foreach (var item in reader.ReadAllAsync(ct).ConfigureAwait(false))
                 {