Use ambient behavior pipeline factory in tests

src/Stunts.UnitTests/BehaviorPipelineTests.cs

@@ -389,13 +389,13 @@ public async Task WhenRunningParallel_ThenCanReplaceLocalPipelineFactory()
             await Task.WhenAll(
                 Task.Run(() =>
                 {
-                    BehaviorPipelineFactory.LocalDefault = factory1;
+                    using var ambient = BehaviorPipelineFactory.UseAmbient(factory1);
                     Thread.Sleep(50);
                     Assert.Same(factory1, BehaviorPipelineFactory.Default);
                 }),
                 Task.Run(() =>
                 {
-                    BehaviorPipelineFactory.LocalDefault = factory2;
+                    using var ambient = BehaviorPipelineFactory.UseAmbient(factory2);
                     Thread.Sleep(50);
                     Assert.Same(factory2, BehaviorPipelineFactory.Default);
                 })

src/Stunts.UnitTests/Castle/ClassProxyTests.cs

@@ -144,18 +144,10 @@ public void ParamsConstructorAcceptsAnArray()
 
         public void VirtualCallDuringConstructionUsesThePipelineFactory()
         {
-            var previous = BehaviorPipelineFactory.LocalDefault;
-            BehaviorPipelineFactory.LocalDefault = new CtorBehaviorFactory();
-            try
-            {
-                var stunt = Stunt.Of<CtorCaller>();
+            using var ambient = BehaviorPipelineFactory.UseAmbient(new CtorBehaviorFactory());
+            var stunt = Stunt.Of<CtorCaller>();
 
-                Assert.Equal("proxy", stunt.Seen);
-            }
-            finally
-            {
-                BehaviorPipelineFactory.LocalDefault = previous;
-            }
+            Assert.Equal("proxy", stunt.Seen);
         }
 
         public void NestedClassAndCharReturn()

src/Stunts.UnitTests/DynamicStuntFactoryTests.cs

@@ -78,7 +78,7 @@ public void DelegateWithoutImplementationThrows()
         [Fact]
         public void ConstructorInterceptionNotSupported()
         {
-            BehaviorPipelineFactory.LocalDefault = new RecordingBehaviorPipelineFactory();
+            using var ambient = BehaviorPipelineFactory.UseAmbient(new RecordingBehaviorPipelineFactory());
             StuntFactory.LocalDefault = new DynamicStuntFactory();
 
             var calculator = Stunt.Of<ICalculator>();

src/Stunts.UnitTests/Scenarios/InterfaceBase.cs

@@ -17,7 +17,7 @@ public class Test : IRunnable
     {
         public void Run()
         {
-            BehaviorPipelineFactory.LocalDefault = new RecordingBehaviorPipelineFactory();
+            using var ambient = BehaviorPipelineFactory.UseAmbient(new RecordingBehaviorPipelineFactory());
             var stunt = Stunt.Of<IBasicInterface>();
 
             Assert.NotNull(stunt);

src/Stunts/BehaviorPipelineFactory.cs

@@ -1,9 +1,10 @@
+using System;
 using System.Threading;
 
 namespace Stunts
 {
     /// <summary>
     /// Provides the global <see cref="Default"/> and <see cref="LocalDefault"/> 
     /// behavior pipeline factory used when creating new stunts.
     /// </summary>
     /// <remarks>
@@ -24,8 +25,8 @@
         /// for creating the initial pipelines used during a stunt instantiation.
         /// </summary>
         /// <remarks>
-        /// A <see cref="LocalDefault"/> can override the value of this global 
-        /// default, if assigned to a non-null value.
+        /// An ambient <see cref="IBehaviorPipelineFactory"/> can override the value of this global 
+        /// default, via <see cref="UseAmbient(IBehaviorPipelineFactory)"/>. This is typically only needed for testing.
         /// </remarks>
         public static IBehaviorPipelineFactory Default
         {
@@ -34,14 +35,24 @@
         }
 
         /// <summary>
-        /// Gets or sets the <see cref="IBehaviorPipelineFactory"/> to use 
-        /// in the current (async) flow, so it does not affect other threads/flows.
-        /// This is typically used in tests to isolate the default pipeline configurations.
+        /// Sets an ambient <see cref="IBehaviorPipelineFactory"/> to use for <see cref="Default"/> for the current execution context.
         /// </summary>
-        public static IBehaviorPipelineFactory? LocalDefault
+        /// <param name="factory">The <see cref="IBehaviorPipelineFactory"/> to set as the ambient <see cref="Default"/>.</param>
+        /// <returns>An <see cref="IDisposable"/> that, when disposed, restores the original execution context default.</returns>
+        public static IDisposable UseAmbient(IBehaviorPipelineFactory factory) => new AmbientDisposable(localFactory, factory);
+
+        class AmbientDisposable : IDisposable
         {
-            get => localFactory.Value;
-            set => localFactory.Value = value;
+            readonly AsyncLocal<IBehaviorPipelineFactory?> storage;
+            readonly IBehaviorPipelineFactory? original;
+
+            public AmbientDisposable(AsyncLocal<IBehaviorPipelineFactory?> storage, IBehaviorPipelineFactory factory)
+            {
+                this.storage = storage;
+                original = storage.Value;
+                storage.Value = factory;
+            }
+            public void Dispose() => storage.Value = original;
         }
 
         class DefaultBehaviorPipelineFactory : IBehaviorPipelineFactory