Stop IntegrationContext from disposing a class fixture it doesn't own (GH-3423)

src/Testing/CoreTests/Configuration/find_handlers_with_the_default_handler_discovery.cs

@@ -17,7 +17,6 @@ public class find_handlers_with_the_default_handler_discovery : IntegrationConte
     public find_handlers_with_the_default_handler_discovery(DefaultApp @default, ITestOutputHelper output) : base(@default)
     {
         _output = output;
-        @default.RecycleIfNecessary();
     }
 
     [Fact]

src/Testing/CoreTests/IntegrationContext.cs

@@ -45,16 +45,7 @@ public DefaultApp()
 
     public void Dispose()
     {
-        Host?.Dispose();
-        Host = null!;
-    }
-
-    public void RecycleIfNecessary()
-    {
-        if (Host == null)
-        {
-            Host = WolverineHost.Basic();
-        }
+        Host.Dispose();
     }
 
     public HandlerChain ChainFor<T>()
@@ -67,10 +58,13 @@ public class IntegrationContext : IDisposable, IClassFixture<DefaultApp>
 {
     private readonly DefaultApp _default;
 
+    // Only set when a test opts out of the shared fixture via with(). That host belongs to the
+    // test and has to be disposed with it; the DefaultApp fixture does not, and must not be.
+    private IHost? _ownedHost;
+
     public IntegrationContext(DefaultApp @default)
     {
         _default = @default;
-        _default.RecycleIfNecessary();
 
         Host = _default.Host;
     }
@@ -84,12 +78,21 @@ public IntegrationContext(DefaultApp @default)
 
     public virtual void Dispose()
     {
-        _default.Dispose();
+        // Dispose only what this test built for itself. DefaultApp is an IClassFixture, so xUnit
+        // owns its lifetime and disposes it once the class finishes. Tearing it down here — after
+        // every test method — left the *shared* host disposed for the tests that followed, and the
+        // guard that papered over that silently rebuilt it as a WolverineHost.Basic() carrying none
+        // of DefaultApp's handler/service registrations. Tests then passed or failed on ordering.
+        // See GH-3423.
+        _ownedHost?.Dispose();
     }
 
     protected async Task with(Action<WolverineOptions> configuration)
     {
-        Host = await WolverineHost.ForAsync(configuration);
+        _ownedHost?.Dispose();
+
+        _ownedHost = await WolverineHost.ForAsync(configuration);
+        Host = _ownedHost;
     }
 
     protected HandlerChain chainFor<T>()