#3324 moved the WebUI session/delivery context from a prefill user message into
_webui_ephemeral_system_prompt, but the gateway-backed WebUI chat path built its
own system message from the bare _WEBUI_PROGRESS_PROMPT + prefill helper — which
now returns only recall prefill. That silently dropped connected platforms, home
channels, delivery hints, and session framing on Gateway-routed WebUI chats
(same #3278 data-drop class, on the path the author missed). Build the gateway
request's system message with _webui_ephemeral_system_prompt(surface_context,
config_data=cfg) so the context is preserved. Regression test asserts the
delivery context is present in the gateway system message.
8ad0db5dbe