feat: enable stream_event forwarding for serve mode streaming

QueryEngine previously guarded stream_event yields behind includePartialMessages,
which was undefined in serve mode. Remove the guard so stream_event messages
always flow to the child process stdout. Add stream_event handling in
sessionHandle.routeMessage to emit via EventBus for downstream consumers.

src/QueryEngine.ts

@@ -837,14 +837,12 @@ export class QueryEngine {
             )
           }
 
-          if (includePartialMessages) {
-            yield {
-              type: 'stream_event' as const,
-              event,
-              session_id: getSessionId(),
-              parent_tool_use_id: null,
-              uuid: randomUUID(),
-            }
+          yield {
+            type: 'stream_event' as const,
+            event,
+            session_id: getSessionId(),
+            parent_tool_use_id: null,
+            uuid: randomUUID(),
           }
 
           break

src/server/sessionHandle.ts

@@ -521,6 +521,11 @@ export class SessionHandle {
         this.pendingQuestions.delete(cancelRequestId)
         break
       }
+      case 'stream_event': {
+        this.lastActiveAt = Date.now()
+        this.emitEvent('stream_event', msg)
+        break
+      }
       default: {
         break
       }