bug(ui): Public chat shows visitors the raw execution error, including the exit code and operator-only remediation #3461

Summary

A red banner with the backend text verbatim: "Execution failed with no output (exit code 1): …" followed by instructions that name the vendor console and an operator Settings page.

Category: functional · Severity: Medium · Validated: source

Context

Found by the click-through UI sweep of a local instance (dev @ c90c711ee, 1.0.0-rc1), fixture agents only.

Repro

1. Open a public chat link while the agent cannot run (for example the model credential is exhausted).
2. Send one message.

Expected

"graceful fallback" — a visitor sees a neutral "can't respond right now" line, as the Workspace shows for the same failure.

Observed

A red banner with the backend text verbatim: "Execution failed with no output (exit code 1): …" followed by instructions that name the vendor console and an operator Settings page.

Validation

src/frontend/src/views/PublicChat.vue:871 and :880 — chatError.value = execution.error || … renders the execution's raw error string returned by the public execution-status call.

Acceptance Criteria

• The repro above gives the Expected result at 1440 and 768, in light and dark.
• A regression test pins the behaviour (unit or Playwright, whichever tier can observe it).
• No new raw-palette classes or hand-rolled primitives are introduced by the fix (docs/memory/design-system-contract.md).

Technical Notes

Evidence screenshots (kept on the machine that ran the sweep; the evidence folder is gitignored because the repository is public):

• docs/testing/ui-sweep/evidence/2026-10-09-all/bb3125b8-1.png

Found by /ui-sweep --all @ c90c711