collab wait: treat empty statuses payload as failed

Summary

When CollabWaitingEnd arrives with an empty statuses payload, downstream UIs/consumers can interpret this as success or completion, which is misleading.

Proposed behavior

Treat empty statuses as a failed wait result.

Rationale

An empty status map indicates missing/incomplete wait results, not successful completion.

Prior PR context

This was previously proposed in PR #12532 (now closed due contribution policy), including implementation and regression tests.

Affected areas

• app-server-protocol thread history reconstruction
• app-server v2 item notifications
• exec JSON event processing