Revert all changes - existing DisableParallelization collection is sufficient per xUnit documentation

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

tests/ModelContextProtocol.Tests/DiagnosticTests.cs

@@ -9,7 +9,7 @@
 
 namespace ModelContextProtocol.Tests;
 
-[Collection(nameof(ExclusiveExecution))]
+[Collection(nameof(DisableParallelization))]
 public class DiagnosticTests
 {
     [Fact]

tests/ModelContextProtocol.Tests/TestAttributes.cs

@@ -1,33 +1,9 @@
-/// <summary>
-/// Test collection for tests that require exclusive execution (no parallel execution with any other tests).
-/// This is needed for tests that use OpenTelemetry TracerProvider with the global static ActivitySource.
-/// Without this, the TracerProvider would collect activities from ALL tests running in parallel.
-/// </summary>
-/// <remarks>
-/// The fixture ensures mutual exclusion across ALL tests by using a static semaphore.
-/// Tests in this collection will acquire the semaphore before running and release it after,
-/// preventing any other test from running concurrently.
-/// </remarks>
-[CollectionDefinition(nameof(ExclusiveExecution))]
-public sealed class ExclusiveExecution : ICollectionFixture<ExclusiveExecutionFixture>;
+// Uncomment to disable parallel test execution for the whole assembly
+//[assembly: CollectionBehavior(DisableTestParallelization = true)]
 
 /// <summary>
-/// Fixture that ensures only one test runs at a time across the entire test assembly.
+/// Enables test classes to individually be attributed as [Collection(nameof(DisableParallelization))]
+/// to have those tests run non-concurrently with any other tests.
 /// </summary>
-public sealed class ExclusiveExecutionFixture : IAsyncLifetime
-{
-    private static readonly SemaphoreSlim s_exclusiveLock = new(1, 1);
-
-    public async ValueTask InitializeAsync()
-    {
-        // Acquire the lock before any test in this collection starts
-        await s_exclusiveLock.WaitAsync();
-    }
-
-    public ValueTask DisposeAsync()
-    {
-        // Release the lock after the test completes
-        s_exclusiveLock.Release();
-        return default;
-    }
-}
+[CollectionDefinition(nameof(DisableParallelization), DisableParallelization = true)]
+public sealed class DisableParallelization;