tui: make empty collab wait status messaging explicit

Summary

In TUI, a completed wait event with no returned statuses currently reads like an in-progress state.

Proposed behavior

Use explicit warning copy when no statuses are returned, e.g. "No agent statuses reported".

Rationale

This aligns presentation with wait semantics and avoids user confusion.

Prior PR context

This was previously proposed in PR #12537 (now closed due contribution policy), with a unit test for the display behavior.