[Bug] Assistant chat output collapses meaningful single newlines

Assistant chat output collapses meaningful single newlines

Description

Assistant messages can contain meaningful single newlines, especially output forwarded from CLI harnesses. The stored transcript preserves those newlines, but the chat view renders them with the default CommonMark behavior and collapses each single newline to a space.

Expected: the chat view preserves the assistant message's line structure while retaining the existing GitHub-flavored Markdown behavior.

Actual: adjacent lines are displayed as one wrapped paragraph, which can change the meaning of task status and handoff output.

Steps to reproduce

1. Persist an assistant message containing Current task: complete\nNext task: review the draft\nFinal task: publish after approval.
2. Open the session in the web chat view.
3. Inspect the assistant message.
4. Observe that the stored newlines are collapsed and no <br> elements are rendered between the lines.

Version

upstream/main@44378d68869120134d0a2663277e38af108f625d

OS

Shared web UI; reproduced with a controlled Chromium fixture.

Harness

Not applicable. The shared assistant Markdown renderer is affected regardless of harness.

Harness mode

Not applicable.

Platform or device

Other: browser web UI.

Observed impact

Some users or sessions.

Authentication type

Not authentication-related.