feat: report a WAC task failure the workflow body never awaited (#11017)

* feat: warn when a WAC task fails and the body never awaited it

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6FHYvk7z4eGvhZXKZB9JF

* fix: report unawaited WAC failures on the failing round and in stream order

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6FHYvk7z4eGvhZXKZB9JF

* docs: state the WAC warn-placement invariant where the wrapper enforces it

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6FHYvk7z4eGvhZXKZB9JF

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>