Preserve IOutboundParameterTransformer on optional route constraints

When a route constraint implements IOutboundParameterTransformer,
wrapping it in OptionalRouteConstraint loses the transformer capability
because OptionalRouteConstraint does not implement the interface.

Add OptionalOutboundParameterTransformerRouteConstraint that extends
OptionalRouteConstraint and delegates TransformOutbound to the inner
constraint. DefaultParameterPolicyFactory.InitializeRouteConstraint
now uses this wrapper when the inner constraint is a transformer.

Fixes #23063

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kubaflo

src/Http/Routing/src/Constraints/OptionalOutboundParameterTransformerRouteConstraint.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.AspNetCore.Routing.Constraints;
+
+/// <summary>
+/// Defines a constraint on an optional parameter whose inner constraint also implements
+/// <see cref="IOutboundParameterTransformer"/>. This preserves the transformer capability
+/// that would otherwise be lost when wrapping in <see cref="OptionalRouteConstraint"/>.
+/// </summary>
+internal sealed class OptionalOutboundParameterTransformerRouteConstraint : OptionalRouteConstraint, IOutboundParameterTransformer
+{
+    /// <summary>
+    /// Creates a new <see cref="OptionalOutboundParameterTransformerRouteConstraint"/> instance.
+    /// </summary>
+    /// <param name="innerConstraint">The inner constraint that also implements <see cref="IOutboundParameterTransformer"/>.</param>
+    public OptionalOutboundParameterTransformerRouteConstraint(IRouteConstraint innerConstraint)
+        : base(innerConstraint)
+    {
+    }
+
+    /// <inheritdoc />
+    public string? TransformOutbound(object? value)
+    {
+        return ((IOutboundParameterTransformer)InnerConstraint).TransformOutbound(value);
+    }
+}

src/Http/Routing/src/DefaultParameterPolicyFactory.cs

@@ -64,7 +64,9 @@ private static IParameterPolicy InitializeRouteConstraint(
     {
         if (optional)
         {
-            routeConstraint = new OptionalRouteConstraint(routeConstraint);
+            routeConstraint = routeConstraint is IOutboundParameterTransformer
+                ? new OptionalOutboundParameterTransformerRouteConstraint(routeConstraint)
+                : new OptionalRouteConstraint(routeConstraint);
         }
 
         return routeConstraint;

src/Http/Routing/test/UnitTests/DefaultParameterPolicyFactoryTest.cs

@@ -127,6 +127,26 @@ public void Create_CreatesParameterPolicy_FromRoutePattern_Constraint_Optional()
         Assert.IsType<IntRouteConstraint>(optionalConstraint.InnerConstraint);
     }
 
+    [Fact]
+    public void Create_CreatesParameterPolicy_FromRoutePattern_Constraint_Optional_PreservesOutboundTransformer()
+    {
+        // Regression test for https://github.com/dotnet/aspnetcore/issues/23063
+        var factory = GetParameterPolicyFactory();
+
+        var parameter = RoutePatternFactory.ParameterPart(
+            "id",
+            @default: null,
+            parameterKind: RoutePatternParameterKind.Optional,
+            parameterPolicies: new[] { RoutePatternFactory.ParameterPolicy(new TransformingRouteConstraint()), });
+
+        var parameterPolicy = factory.Create(parameter, parameter.ParameterPolicies[0]);
+
+        var optionalConstraint = Assert.IsType<OptionalOutboundParameterTransformerRouteConstraint>(parameterPolicy);
+        Assert.IsType<TransformingRouteConstraint>(optionalConstraint.InnerConstraint);
+        var transformer = Assert.IsAssignableFrom<IOutboundParameterTransformer>(parameterPolicy);
+        Assert.Equal("hello", transformer.TransformOutbound("HELLO"));
+    }
+
     [Fact]
     public void Create_CreatesParameterPolicy_FromRoutePattern_ParameterPolicy()
     {
@@ -445,6 +465,24 @@ public bool Match(
             return false;
         }
     }
+
+    private class TransformingRouteConstraint : IRouteConstraint, IOutboundParameterTransformer
+    {
+        public bool Match(
+            HttpContext httpContext,
+            IRouter route,
+            string routeKey,
+            RouteValueDictionary values,
+            RouteDirection routeDirection)
+        {
+            return true;
+        }
+
+        public string TransformOutbound(object value)
+        {
+            return value?.ToString()?.ToLowerInvariant();
+        }
+    }
 }
 
 public class CustomParameterPolicy : IParameterPolicy