ListRecord.Field array-shaped message still fails in the production GsonFactory (follow-up to #1587) #1653

Issue summary

When the Slack API returns a conversations.replies (or conversations.history) response that contains a Slack Lists item attachment, the SDK throws JsonSyntaxException whenever any ListRecord.Field.message value is a JSON array instead of a single object. The whole API call fails, and no Message data is returned.

This is the bug from #1587. #1590 (released in v1.49.0) closed #1587, but the bug is still in the production code path in v1.52.0.

Affected versions

• v1.46.0 through v1.48.1 (verified in ListRecord.Field.message` deserialization fails when Slack returns an array #1587; also seen in production on v1.48.1)
• v1.49.0 through v1.52.0: the production GsonFactory has no adapter for ListRecord.Field. @maoppay saw the failure on v1.50.0 (comment in ListRecord.Field.message` deserialization fails when Slack returns an array #1587), and the live probe in test: probe ListRecord.Field message wire shape against live API (experimental) #1644 shows it too.

Why #1590 was not sufficient

#1590 changed two things:

1. ListRecord.Field (slack-api-model/src/main). It added private transient List<Message> messages and getMessages() / setMessages(). The field message is still a single Message. Gson skips transient fields and sets fields by reflection, not through setters. So without an adapter, Gson still maps the JSON key message to the Message field, and an array still fails.
2. GsonListRecordFieldFactory, the adapter that converts the array. fix(model): handle array format for ListRecord.Field message deserial… #1590 put it in slack-api-model/src/test/java/test_locally/util/list/ and registered it only in the test GsonFactory (slack-api-model/src/test/java/test_locally/unit/GsonFactory.java).

ListRecordFieldTest parses with that test GsonFactory, so it passes. Real API calls parse with com.slack.api.util.json.GsonFactory in slack-api-client (MethodsClientImpl calls GsonFactory.createSnakeCase(config)), and that class does not register the adapter. Also, the json-logs/samples/api samples still have message as a single object, so the MockSlackApi tests (for example ConversationsTest) never see the array shape.

Minimal reproduction

import com.google.gson.Gson;
import com.slack.api.methods.response.conversations.ConversationsRepliesResponse;
import com.slack.api.util.json.GsonFactory;

public class Repro {
    public static void main(String[] args) {
        // Mirrors the shape Slack returns when a Lists item's `message` field
        // has multiple values (array form).
        String payload =
            "{\"ok\":true,\"messages\":[{\"type\":\"message\",\"user\":\"U1\",\"ts\":\"1.0\","
            + "\"text\":\"hi\",\"attachments\":[{\"list_record\":{\"record\":{\"fields\":["
            + "{\"key\":\"name\",\"message\":{\"text\":\"ok\"}},"
            + "{\"key\":\"details\",\"message\":[{\"text\":\"a\"},{\"text\":\"b\"}]}"
            + "]}}}]}]}";

        Gson sdkGson = GsonFactory.createSnakeCase();
        // Still throws JsonSyntaxException on v1.52.0 at
        // $.messages[0].attachments[0].list_record.record.fields[1].message
        ConversationsRepliesResponse parsed =
            sdkGson.fromJson(payload, ConversationsRepliesResponse.class);
    }
}

Expected behavior

The SDK accepts both single-object and array shapes for ListRecord.Field.message in real API responses, as getMessages() from #1590 intends.

Proposed fix

#1654 moves GsonListRecordFieldFactory to slack-api-model/src/main/java/com/slack/api/util/json/ (next to GsonListViewGroupingFactory) and registers it in GsonFactory.registerTypeAdapters.