Make FieldMaskTree paths that have no other type they are corecursive with (message or descriptor) avoid blowing the C++ stack.

This improves behavior when reached with arbitrarily long field paths.

Cases that are bounded by a Message* or Descriptor* depth are not expected to be protected; those cases we enforce depth limits at parse time and it is up to the caller to handle depth risks of in-memory constructed objects.

https://github.com/protocolbuffers/protobuf/issues/26489

PiperOrigin-RevId: 892562508