Fix: Mark catch-all route parameters as optional (#60392)

- Added a unit test in DefaultApiDescriptionProviderTest to validate that catch-all route parameters (e.g., "{**catchAllParameter}") are reported as optional.
- Modified ProcessIsRequired in DefaultApiDescriptionProvider to detect catch-all parameters (by checking for the "{**" pattern in the route template) and mark them as not required.
- This change aligns ApiExplorer metadata with the actual runtime behavior, ensuring more accurate API documentation.
- Follows TDD practices by first writing a failing test and then implementing the fix.

Author: mapedersen

src/Mvc/Mvc.ApiExplorer/src/DefaultApiDescriptionProvider.cs

@@ -315,9 +315,18 @@ internal static void ProcessIsRequired(ApiParameterContext context, MvcOptions m
                 parameter.IsRequired = true;
             }
 
-            if (parameter.Source == BindingSource.Path && parameter.RouteInfo != null && !parameter.RouteInfo.IsOptional)
+            if (parameter.Source == BindingSource.Path && parameter.RouteInfo != null)
             {
-                parameter.IsRequired = true;
+                // If the route template contains a catch-all parameter marker ("{**"), treat it as optional.
+                var template = context.ActionDescriptor.AttributeRouteInfo?.Template;
+                if (!string.IsNullOrEmpty(template) && template.Contains("{**"))
+                {
+                    parameter.IsRequired = false;
+                }
+                else if (!parameter.RouteInfo.IsOptional)
+                {
+                    parameter.IsRequired = true;
+                }
             }
         }
     }

src/Mvc/Mvc.ApiExplorer/test/DefaultApiDescriptionProviderTest.cs

@@ -32,6 +32,23 @@ namespace Microsoft.AspNetCore.Mvc.Description;
 
 public class DefaultApiDescriptionProviderTest
 {
+    [Fact]
+    public void CatchAllParameter_IsReportedAsOptional()
+    {
+        // Arrange: Create an action descriptor with a catch-all route template.
+        var action = CreateActionDescriptor();
+        action.AttributeRouteInfo = new AttributeRouteInfo { Template = "{**catchAllParameter}" };
+
+        // Act: Get the API descriptions using the existing helper.
+        var descriptions = GetApiDescriptions(action);
+
+        // Assert: There should be one description, with one parameter named "catchAllParameter"
+        // and its IsRequired property should be false.
+        var description = Assert.Single(descriptions);
+        var parameter = Assert.Single(description.ParameterDescriptions, p => p.Name == "catchAllParameter");
+        Assert.False(parameter.IsRequired);
+    }
+
     [Fact]
     public void GetApiDescription_IgnoresNonControllerActionDescriptor()
     {