Handle groups in C# JsonParser (#16970)

Fixes https://github.com/protocolbuffers/protobuf/issues/16968

Closes #16970

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/16970 from CommonGuy:main b008e18934
PiperOrigin-RevId: 638578696

csharp/src/Google.Protobuf.Test/JsonParserTest.cs

@@ -995,6 +995,16 @@ namespace Google.Protobuf
             Assert.AreEqual(0, parsed.FieldName13);
         }
 
+        [Test]
+        public void Proto2_Group()
+        {
+            string json = "{ \"data\": { \"groupInt32\": 2, \"groupUint32\": 3 } }";
+            var parsed = TestAllTypesProto2.Parser.ParseJson(json);
+            Assert.True(parsed.HasData);
+            Assert.AreEqual(2, parsed.Data.GroupInt32);
+            Assert.AreEqual(3, parsed.Data.GroupUint32);
+        }
+
         [Test]
         [TestCase("5")]
         [TestCase("\"text\"")]

csharp/src/Google.Protobuf/JsonParser.cs

@@ -330,7 +330,7 @@ namespace Google.Protobuf
             }
 
             var fieldType = field.FieldType;
-            if (fieldType == FieldType.Message)
+            if (fieldType == FieldType.Message || fieldType == FieldType.Group)
             {
                 // Parse wrapper types as their constituent types.
                 // TODO: What does this mean for null?