Fix TUI stream cleanup after turn errors

[etraut-openai]

Summary

Fixes #22726.

After a Responses stream disconnect, the live TUI could keep accepting prompts while leaving partially streamed assistant output in its transient streaming-cell form. That made fenced diffs or SVG/XML-like content appear as raw transcript text until the user closed the TUI and resumed the same session, which rebuilt the transcript from saved history.

This change finalizes the active answer stream before generic failed-turn cleanup clears the stream controller, so the live transcript takes the same source-backed markdown consolidation path as a successful turn.

Reviewer repro

1. Start a local Codex TUI session.
2. Trigger an assistant turn that streams markdown content, especially a fenced diff or SVG/XML-like block.
3. Force or encounter a non-retry stream disconnect before the turn completes.
4. Continue using the same still-open TUI session.
5. Before this fix, the live history can stay raw/plain even though codex resume renders the same session normally.
6. After this fix, the failed-turn path consolidates the partial stream before rendering the error, so the live TUI keeps normal transcript rendering.

[fcoury-oai]

Some tested with a custom mock python server that triggers a stream disconnected before completion and it handled it gracefully.

Approved 👍