Fix flaky DiagnosticTests by clearing Activity.Current and forcing flush

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Tests/DiagnosticTests.cs

@@ -15,6 +15,9 @@ public class DiagnosticTests
     [Fact]
     public async Task Session_TracksActivities()
     {
+        // Ensure no ambient activity from previous tests
+        Activity.Current = null;
+
         var activities = new List<Activity>();
         var clientToServerLog = new List<string>();
 
@@ -32,6 +35,9 @@ await RunConnected(async (client, server) =>
                 var tool = tools.First(t => t.Name == "DoubleValue");
                 await tool.InvokeAsync(new() { ["amount"] = 42 }, TestContext.Current.CancellationToken);
             }, clientToServerLog);
+
+            // Ensure all activities are flushed before disposal
+            tracerProvider.ForceFlush();
         }
 
         Assert.NotEmpty(activities);
@@ -77,6 +83,9 @@ await RunConnected(async (client, server) =>
     [Fact]
     public async Task Session_FailedToolCall()
     {
+        // Ensure no ambient activity from previous tests
+        Activity.Current = null;
+
         var activities = new List<Activity>();
 
         using (var tracerProvider = OpenTelemetry.Sdk.CreateTracerProviderBuilder()
@@ -89,6 +98,9 @@ await RunConnected(async (client, server) =>
                 await client.CallToolAsync("Throw", cancellationToken: TestContext.Current.CancellationToken);
                 await Assert.ThrowsAsync<McpProtocolException>(async () => await client.CallToolAsync("does-not-exist", cancellationToken: TestContext.Current.CancellationToken));
             }, []);
+
+            // Ensure all activities are flushed before disposal
+            tracerProvider.ForceFlush();
         }
 
         Assert.NotEmpty(activities);