Session defect errors are not surfaced or rendered concisely

[DatScreamer]

Description

Unexpected session defects have two user-visible failure modes: before propagation, the session goes idle without an error; after propagation is fixed, the UI can show the entire stack trace in the error card. The complete fix must propagate the error before idle, persist it on the assistant message, and show a concise message while retaining full details in logs.

This is the follow-up acceptance issue for the end-to-end behavior addressed by PR #42516, which includes the propagation work from #42253.

Plugins

None required.

OpenCode version

1.18.18

Steps to reproduce

1. Open a project in the web UI.
2. Create a session.
3. Trigger a session defect, such as sending a prompt after the workspace path disappears.
4. Observe the session timeline.

Expected behavior

The defect reaches the UI before the session becomes idle, is persisted on the assistant message, and appears as a concise error card. The full cause remains available in server logs.

Operating System

Linux

[DatScreamer]

PR #39200 overlaps the assistant-message finalization and concise-error portion of this issue. It does not cover the propagation-before-idle or tool-warning work from #42253. PR #42516 contains #42253 plus the follow-up persistence and formatting changes; keeping this issue open while maintainers choose whether to port #39200 or merge the current-architecture implementation.