Add ActiveIssue for SendAsync_Success_ConnectionSetupActivityGraphRecorded (#129886)

`DiagnosticsTest.SendAsync_Success_ConnectionSetupActivityGraphRecorded`
fails intermittently.

### Changes
- Annotate the test with
`[ActiveIssue("https://github.com/dotnet/runtime/issues/129771")]` in
`DiagnosticsTests.cs` to skip it while the underlying race is
investigated.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>

Author: Copilot

src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs

@@ -577,6 +577,7 @@ protected static void VerifyTag<T>(KeyValuePair<string, object?>[] tags, string
             }
         }
 
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129771")]
         [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
         [InlineData(false)]
         [InlineData(true)]