fix: auto-complete previous pending messages when a new assistant message starts

When the agent produces multiple intermediate responses during a turn,
each gets a unique messageId but all stay pending until finalizeTurn().
This caused multiple stacked 'Thinking' bubbles in the chat transcript.

Now, when a new messageId arrives in applyTurnDelta, any previously
pending assistant messages are marked complete and message-complete
events are emitted before the new message-delta. The renderer reducer
mirrors this logic for defensive consistency.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/lib/sessionWorkspace.ts

@@ -112,10 +112,18 @@ function applyMessageDeltaEvent(session: SessionRecord, event: SessionEventRecor
     };
   }
 
+  // Auto-complete any previously pending assistant messages so only
+  // the new message shows the "Thinking" indicator.
+  const completedMessages = session.messages.map((message) =>
+    message.pending && message.role === 'assistant'
+      ? { ...message, pending: false }
+      : message,
+  );
+
   return {
     ...session,
     messages: [
-      ...session.messages,
+      ...completedMessages,
         {
           id: event.messageId,
           role: 'assistant',