Consecutive user turns (session-context prefill + actual message) made models
with strict chat templates (Mistral, Gemma via llama.cpp) reject the request
with a Jinja 500. Move the platform/delivery context (connected platforms, home
channels, scheduled-task delivery hints) from _webui_session_context_message
(a prefill user message) into _webui_delivery_context_prompt, appended to the
ephemeral system prompt. Session framing stays in _webui_surface_context_prompt.
Context is preserved — just role-alternation-safe. Closes#3276.
Co-authored-by: aether-agent <aether-agent@users.noreply.github.com>
