[BUG] Flaky restart-persistence E2E: TaskMessagesReadError on ui_messages.json after restart #2078

Summary

The mocked restart-persistence E2E test failed once in about 100 recent runs. The verify phase could not read ui_messages.json for a task that was in history.

Evidence

• Run: https://github.com/Zoo-Code-Org/Zoo-Code/actions/runs/38998169341 (job 117295222479), PR [Fix] Fork PR review-state labels stay stale until the hourly sweep #2049.
• Failed step: "Run mocked restart-persistence E2E test".
• create phase passed in about 1 second, then quit VS Code.
• verify phase logged:
  [createTaskWithHistoryItem] taskScheduler.schedule failed: TaskMessagesReadError: Failed to read task messages for 01a147e2-1838-74dd-8726-1b9b23e29901 at .../tasks/01a147e2-.../ui_messages.json
• Stack: readTaskMessages <- Task.resumeTaskFromHistory <- TaskScheduler.schedule.
• The test then waited until its 30 second timeout.
• The PR changes only a workflow file and its test. It does not touch task persistence.
• Of the last 100 runs of "E2E Tests (Mocked)", this is the only one with this error.

Cause (not confirmed)

Leading guess: create quits right after TaskCompleted. The final saveClineMessages write may not be on disk yet.

• waitForMarkedCompletion checks API conversation history. It does not check ui_messages.json.
• readFileWithMissingRetry retries a missing file once, after 1 to 10 ms. This does not cover a late write.
• readTaskMessages stores the real error in originalError only. The log message does not show whether the file was missing, unreadable or half-written.

Proposed work

1. Include the underlying error code in the TaskMessagesReadError message.
2. Make the create phase wait until ui_messages.json contains the completion marker before it quits.
3. Re-check the retry window in readFileWithMissingRetry if step 2 does not remove the flake.

Related

• Recover partial TaskHistoryStore.atomicUpdatePair lifecycle writes #1540 (partial TaskHistoryStore.atomicUpdatePair writes) is nearby but covers a different file.