address review: document statelessness + tighten opt-out assertion

- Document that both handler types inserted by TUnitHttpClientFilter must
  remain stateless/thread-safe because IHttpClientFactory caches pipelines
  and shares handler instances across concurrent parallel-test requests.
- Explain the outermost-insert intent so a future refactor doesn't reverse
  the order.
- Opt-out test now also asserts the downstream hop does not carry baggage,
  mirroring the positive test.

TUnit.AspNetCore.Core/Http/TUnitHttpClientFilter.cs

@@ -7,13 +7,23 @@ namespace TUnit.AspNetCore.Http;
 /// to every <see cref="System.Net.Http.IHttpClientFactory"/> handler pipeline built in the SUT.
 /// Ensures outbound HTTP calls made via <c>AddHttpClient&lt;T&gt;()</c>, named, or typed clients
 /// carry the current test's <c>traceparent</c>, <c>baggage</c>, and <c>X-TUnit-TestId</c> headers.
+/// <para>
+/// Both handler types must remain stateless and thread-safe: <see cref="System.Net.Http.IHttpClientFactory"/>
+/// caches the built pipeline and shares the same handler instances across every request on a given
+/// named client, including concurrent requests from parallel tests. Per-test correlation comes from
+/// <see cref="TUnit.Core.TestContext.Current"/> and <see cref="System.Diagnostics.Activity.Current"/>,
+/// which are async-local — do not add instance fields capturing per-request state to either handler.
+/// </para>
 /// </summary>
 internal sealed class TUnitHttpClientFilter : IHttpMessageHandlerBuilderFilter
 {
     public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next) =>
         builder =>
         {
             next(builder);
+            // Insert at outermost positions so TUnit headers are emitted before any
+            // SUT-registered handler can run. Order must stay ActivityPropagationHandler
+            // first (writes traceparent/baggage) then TUnitTestIdHandler (writes X-TUnit-TestId).
             builder.AdditionalHandlers.Insert(0, new ActivityPropagationHandler());
             builder.AdditionalHandlers.Insert(1, new TUnitTestIdHandler());
         };

TUnit.AspNetCore.Tests/IHttpClientFactoryPropagationTests.cs

@@ -51,5 +51,6 @@ public async Task SutHttpClientFactory_DoesNotPropagate_WhenAutoPropagationDisab
 
         await Assert.That(echoed).DoesNotContain(TUnitTestIdHandler.HeaderName);
         await Assert.That(echoed).DoesNotContain("traceparent:");
+        await Assert.That(echoed).DoesNotContain("baggage:");
     }
 }