Live-probed user's tenant: three of three external-runtime workspaces
register with delivery_mode = NULL, not "poll". The earlier narrow
poll-only check fell through to the misleading 503 for the actually-
observed shape.
Invariant we want: URL empty + not-exactly-"push" → no dispatch path
will ever exist → 422. Only push-mode with empty URL is genuinely
transient (mid-boot, restart in progress) → 503.
Added TestChatUpload_NullModeEmptyURL using the user's actual workspace
ID. Existing TestChatUpload_NoURL switched to explicit "push" mode
(was relying on default — unsafe given the new branching).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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