[dotnet] Add reflection-based JSON serialization fallback to Commands

Fixes SeleniumHQ#14731

dotnet/src/webdriver/Command.cs

@@ -21,6 +21,7 @@
 using System.Collections.Generic;
 using System.Text.Json;
 using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
 
 namespace OpenQA.Selenium
 {
@@ -35,7 +36,7 @@ public class Command
 
         private readonly static JsonSerializerOptions s_jsonSerializerOptions = new()
         {
-            TypeInfoResolver = CommandJsonSerializerContext.Default,
+            TypeInfoResolver = JsonTypeInfoResolver.Combine(CommandJsonSerializerContext.Default, new DefaultJsonTypeInfoResolver()),
             Converters = { new ResponseValueJsonConverter() }
         };
 

dotnet/test/common/CommandTests.cs

@@ -0,0 +1,22 @@
+using NUnit.Framework;
+using System.Collections.Generic;
+
+namespace OpenQA.Selenium
+{
+    [TestFixture]
+    public class CommandTests
+    {
+        [Test]
+        public void CommandSerializesAnonymousType()
+        {
+            var parameters = new Dictionary<string, object>
+            {
+                ["arg"] = new { param1 = true, param2 = false },
+            };
+
+            var command = new Command(new SessionId("session"), "test command", parameters);
+
+            Assert.That(command.ParametersAsJsonString, Is.EqualTo("""{"arg":{"param1":true,"param2":false}}"""));
+        }
+    }
+}