fix(web-api): type Slack Lists message fieldFix Slack Lists message field response type responses

[cxdmnls]

Summary

Fixes #2598.

This PR updates the Slack Lists FieldMessage.message type to match the API response shape. The field can now be either a single message object or an array of message objects. A focused tsd type test was added to cover both response shapes.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

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[zimeg]

👋 Hey @cxdmnls! Thanks for sending this in!

I'm going to close it to prefer approach more aligned with #2599 or codegen from an adjacent project. Super appreciate the PR 🎁

[zimeg]

🐛 issue: This is breaking to the API input with an internal error shown in #2640