Deep research asks 2-3 clarifying questions first. When the user answers
with a bare affirmation ('yes', 'ok', 'go ahead'), that short message
becomes latest_message and the query-synthesis fallback returned it
verbatim, so research ran on the literal word 'yes'.
In ResearchHandler.synthesize_query, when synthesis can't run (history
too short) or fails, fall back to the earliest substantive user message
(the original ask) only when the latest message is an explicit
affirmation/continuation phrase or is empty/punctuation-only. There is
deliberately no length heuristic: a short answer like 'UK', 'C++', or
'Rust' in a clarification flow is a real topic and is left untouched.
Tests cover query/topic selection: bare 'yes' -> original ask, short
answers (UK, C++) kept, short-only-substantive message kept, and a
multi-word follow-up still flows through synthesis.
