fix(types): include bot_id in thread broadcasts

[sean0407]

Summary

Fixes #2766 by adding the optional top-level bot_id field to ThreadBroadcastMessageEvent. Thread broadcasts can be authored by users or bots, and root.bot_id describes the parent message rather than the broadcast reply. This lets TypeScript consumers distinguish the reply author correctly.

Validation

• npm ci --ignore-scripts --no-audit --no-fund — completed in an isolated Node 20 Docker container.
• npm run build --workspace=@slack/types — passed.
• npm run test:types --workspace=@slack/types — passed, including a type fixture with top-level bot_id.
• git diff --check and the project patch policy — passed; 3 files changed, no binary files.

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.

[changeset-bot]

🦋 Changeset detected

Latest commit: 23acb21

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

This PR includes changesets to release 1 package

Name	Type
@slack/types	Patch

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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @sean0407 to sign the Salesforce Inc. Contributor License Agreement.