_parseDeltaElement's fallback branch called child.text on any
non-Element DOM node, including dom.Comment (whose .text getter
aliases its raw data). This let real HTML comments -- e.g. the
internal tracking marker Google's AI Overview panel injects when
you copy its answer text -- surface as visible garbage in the
editor. Skip non-Text nodes here, matching the dom.Text check
_parseElement already does one level up.
