fix(socket-mode): handle Events API payloads without inner events

[kyungyeon-byun-imweb-me]

Summary

Fixes #2753.

SocketModeClient currently assumes every events_api payload contains payload.event.type. Slack's documented app_rate_limited payload has its event type at the top level and no inner event, so that assumption throws before consumers can acknowledge the envelope.

This change:

• dispatches a valid inner event when present, preserving existing behavior;
• otherwise dispatches a valid top-level payload type, including app_rate_limited;
• skips only the specific event emission for malformed payloads while preserving the generic slack_event emission and ACK function;
• adds focused regression coverage for normal callbacks, app_rate_limited, malformed payloads, and invalid inner-event type fallback;
• includes a patch changeset for @slack/socket-mode.

Validation completed:

• npm run build --workspace=@slack/logger
• npm run build --workspace=@slack/types
• npm run build --workspace=@slack/web-api
• npm run build --workspace=@slack/socket-mode
• npm test --workspace=@slack/socket-mode (43 passing)
• npm run lint

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: 4f1ff57

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

This PR includes changesets to release 1 package

Name	Type
@slack/socket-mode	Patch

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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @kyungyeon-byun-imweb-me to sign the Salesforce Inc. Contributor License Agreement.