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

@@ -837,7 +837,6 @@ export class QueryEngine {
             )
           }
 
-          if (includePartialMessages) {
           yield {
             type: 'stream_event' as const,
             event,
@@ -845,7 +844,6 @@ export class QueryEngine {
             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
       }