webview coverage leg can exit 1 with every test passing (vitest worker teardown race with a pending user console log) #2352

Observed twice in CI and three times locally. In both CI cases the suite passed in full and the leg still exited 1.

• PR feat(experiments): make chat-diff the default approval path (L2, #1375) #1384, job/check-run 128576671104 (163,700 B of log): 188 test files and 2,498 tests all passed with zero FAIL lines, exit 1 on EnvironmentTeardownError: [vitest-worker]: Closing rpc while onUserConsoleLog was pending inside @roo-code/vscode-webview:test:coverage. The Windows leg then went cancelled by fail-fast with the annotation The strategy configuration was canceled because "unit-test.ubuntu-latest_ubuntu-late" failed. The leg was green on the previous head of the same branch, and the PR changes webview-ui only.
• PR fix(write-to-file): capture streaming failure once, report it once (split 3/6 of #1066) #1930 reproduced the same error 1 time in 3 local runs of the core shard with every test passing, on the pinned nodejs 22.23.4 from .tool-versions.

The pending console output in the #1384 log is four Uncaught [Error: ...] blocks from webview-ui/src/__tests__/ErrorBoundary*.spec - pre-existing on main, not code either PR touched. The sequence looks like: a test logs through the worker RPC late, teardown closes the RPC while that flush is still pending, vitest reports an unhandled rejection, and the process exits non-zero with a green suite.

Why this needs a decision rather than a re-run: a required context that can go red with a green suite makes the merge gate depend on a race, and re-triggering only re-rolls the dice. Locally it is not deterministic (1 run in 3), so the fair bar for reproducing it is that the suite passed. Visible options: make the ErrorBoundary specs assert or swallow their expected errors so nothing reaches the worker RPC after teardown, or drain pending user-console logs before the coverage project closes the RPC. Nothing has been changed for this; filing so the re-run requests have a permanent reference.