[C#] Conformance test: don't skip JsonIgnoreUnknownParsingTest in runner (#15369)

# Motivation

https://github.com/protocolbuffers/protobuf/issues/7392#issuecomment-1884666885

This is for completeness - I am not blocked by this change in any way, so feel free to close the PR or treat is a low priority.

# Changes

Implement JsonIgnoreUnknownParsingTest in C# conformance test runner.

# Tested

Locally with:

```
bazel test //csharp:conformance_test \
    --action_env=DOTNET_CLI_TELEMETRY_OPTOUT=1 --test_env=DOTNET_CLI_HOME=~ \
    --action_env=DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1
```

Closes #15369

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/15369 from noom:issue-7392/show-csharp-failures 3a652202a0
PiperOrigin-RevId: 602537927

conformance/failure_list_csharp.txt

@@ -13,3 +13,9 @@ Required.Proto3.JsonInput.OneofFieldNullSecond.JsonOutput
 Required.Proto3.JsonInput.OneofFieldNullSecond.ProtobufOutput
 Recommended.Proto3.ValueRejectInfNumberValue.JsonOutput
 Recommended.Proto3.ValueRejectNanNumberValue.JsonOutput
+Recommended.Proto2.JsonInput.IgnoreUnknownEnumStringValueInMapValue.ProtobufOutput
+Recommended.Proto2.JsonInput.IgnoreUnknownEnumStringValueInOptionalField.ProtobufOutput
+Recommended.Proto2.JsonInput.IgnoreUnknownEnumStringValueInRepeatedField.ProtobufOutput
+Recommended.Proto3.JsonInput.IgnoreUnknownEnumStringValueInMapValue.ProtobufOutput
+Recommended.Proto3.JsonInput.IgnoreUnknownEnumStringValueInOptionalField.ProtobufOutput
+Recommended.Proto3.JsonInput.IgnoreUnknownEnumStringValueInRepeatedField.ProtobufOutput

csharp/src/Google.Protobuf.Conformance/Program.cs

@@ -72,11 +72,8 @@ namespace Google.Protobuf.Conformance
                 switch (request.PayloadCase)
                 {
                     case ConformanceRequest.PayloadOneofCase.JsonPayload:
-                        if (request.TestCategory == global::Conformance.TestCategory.JsonIgnoreUnknownParsingTest)
-                        {
-                            return new ConformanceResponse { Skipped = "CSharp doesn't support skipping unknown fields in json parsing." };
-                        }
-                        var parser = new JsonParser(new JsonParser.Settings(20, typeRegistry));
+                        bool ignoreUnknownFields = request.TestCategory == global::Conformance.TestCategory.JsonIgnoreUnknownParsingTest;
+                        JsonParser parser = new JsonParser(new JsonParser.Settings(20, typeRegistry).WithIgnoreUnknownFields(ignoreUnknownFields));
                         message = request.MessageType switch
                         {
                             "protobuf_test_messages.proto3.TestAllTypesProto3" => parser.Parse<ProtobufTestMessages.Proto3.TestAllTypesProto3>(request.JsonPayload),