Change the arena.hpp ::Fuse return type to bool to surface whether the fuse failed.

PiperOrigin-RevId: 628161481

upb/mem/arena.hpp

@@ -26,7 +26,11 @@ class Arena {
 
   upb_Arena* ptr() const { return ptr_.get(); }
 
-  void Fuse(Arena& other) { upb_Arena_Fuse(ptr(), other.ptr()); }
+  // Fuses the arenas together.
+  // This operation can only be performed on arenas with no initial blocks. Will
+  // return false if the fuse failed due to either arena having an initial
+  // block.
+  bool Fuse(Arena& other) { return upb_Arena_Fuse(ptr(), other.ptr()); }
 
  protected:
   std::unique_ptr<upb_Arena, decltype(&upb_Arena_Free)> ptr_;