Agent failures no longer appear as blank replies

What changed

Agent turns that fail or finish without displayable content now show an error instead of an empty assistant reply. In-progress task indicators are also terminalized when their stream is interrupted.

Why this matters to the user

Failed Agent runs no longer look like blank responses or tasks that continue spinning indefinitely.

What the user should do

Nothing — automatic.

Notes for release manager

The underlying provider failure is still reported separately; this change makes the terminal UI state accurate and preserves Agent error diagnosis.