fix(desktop): prevent thinking block from closing mid-streaming

When reasoning text grows during streaming, new parts can be appended
beyond endIndex.  The pending check used slice(startIndex, endIndex)
which excluded these new parts — if the original part completed, the
block would close while new reasoning was still streaming.

Fix: remove the endIndex cap from slice() so all parts from startIndex
onward are checked.  During non-streaming, the array is stable and
all parts are within range anyway.

apps/desktop/src/components/assistant-ui/thread.tsx

@@ -438,7 +438,7 @@ const ReasoningAccordionGroup: FC<{ children?: ReactNode; endIndex: number; star
       s.thread.isRunning &&
       s.message.status?.type === 'running' &&
       s.message.parts
-        .slice(Math.max(0, startIndex), Math.min(s.message.parts.length, endIndex))
+        .slice(Math.max(0, startIndex))
         .some(p => p?.type === 'reasoning' && p.status?.type !== 'complete')
   )