fix(chat): keep reasoning traces visible after the turn finishes

hideReasoning was !working(), so reasoning parts were filtered out of the
transcript the instant the turn went idle — you could only catch them
mid-stream and they vanished on completion. Reasoning already lives inside
the collapsible steps (collapsed by default), so there's no clutter cost
to keeping it: render it regardless of working state so the trace persists
and can be expanded and reviewed after the answer lands.

Author: ishaan1124

frontend/ui/src/components/session-turn.tsx

@@ -660,7 +660,7 @@ export function SessionTurn(
                               message={assistantMessage}
                               responsePartId={responsePartId()}
                               hideResponsePart={hideResponsePart()}
-                              hideReasoning={!working()}
+                              hideReasoning={false}
                               hideTools={props.hideTools}
                             />
                           )}