fix: update SlackListsItemFieldMessage to support single and array message shapes

[swayamsingh2004]

Fixes #2598

Problem

SlackListsItemFieldMessage typed the message field as string[], which was incorrect in two ways:

1. The Slack API can return message as either a single object or an array of objects
2. The elements are message objects (with text, ts, etc.), not strings

Solution

• Added a new SlackListsItemMessage interface with optional fields: text, ts, user, team, type
• Changed message: string[] to message: SlackListsItemMessage | SlackListsItemMessage[] to handle both shapes the API can return

References

• Closes SlackListsItem FieldMessage.message type does not match API response shape #2598
• Same inconsistency reported on Java SDK: ListRecord.Field.message` deserialization fails when Slack returns an array java-slack-sdk#1587

[changeset-bot]

🦋 Changeset detected

Latest commit: f0c9f1e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@slack/web-api	Major
@slack/oauth	Patch
@slack/rtm-api	Patch
@slack/socket-mode	Patch

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[salesforce-cla]

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