With unifiedSession enabled, all turns are routed to the shared
unified:default session, so title generation, persistence, and change
notifications all happen on that shared session. The WebUI sidebar,
however, renders per-chat websocket:<id> sessions, so generated titles
never reach the session the user sees and every chat stays "Untitled".
Pass the origin chat_id through maybe_generate_webui_title_after_turn and
add a target_session_key to maybe_generate_webui_title so the title is
written to the per-chat session while conversation content still comes
from the shared (routed) session. When the routed session differs from
the per-chat session, the latest user/assistant exchange is used as the
title prompt since the shared session mixes content from every channel.
The shared session's title metadata is no longer read or written, so
stale shared titles can no longer leak into per-chat sessions.
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