* [editor-core] avoid duplicate non-bmp check on InsertOp.insertion
* [editor-core] remove non-bmp check from StringFileData
Existing content may contain non-bmp characters. New edits with non-bmp
input are rejected via non-bmp check in InsertOp constructor.
GitOrigin-RevId: e9ef3a4a8c4696e8dc751325608a788351791d1c