Consider DecoratorNode in $insertNodeToNearestRoot

DecoratorNode is not treated as ElementNode from this commit (https://github.com/facebook/lexical/pull/5371) on, but In `$insertNodeToNearestRoot` method, `getTopLevelElementOrThrow` is called if the selection is not a range selection. It causes error while the focus node is a DecoratorNode.

https://github.com/facebook/lexical/blob/686217de2d88a428eae3c7fd4159ef28117e0861/packages/lexical-utils/src/index.ts#L473

And another feature in this method should which I think it should be modified is checking `empty` after splitting node in range selection, otherwise it may create one more empty line.

Another question is why does this method end with inserting another paragraph node?

Read packages/lexical-utils/src/index.ts at $insertNodeToNearestRoot and trace the linked getTopLevelElementOrThrow call through the DecoratorNode and range-selection paths. Reproduce the non-range DecoratorNode failure and the post-split empty-line case; done means both cases behave without the reported error or unintended extra paragraph, and the method’s final insertion behavior is clarified.

Written by the indexing model from the issue text.