buildFullkeyPattern

Builds a pattern that matches the json_tree fullkey of the targeted node, for use as fullkey LIKE ? ESCAPE '\'. SQLite emits a key unquoted only when it is [A-Za-z][A-Za-z0-9]*; otherwise it double-quotes it (e.g. $."user_name"). We reproduce that quoting and escape LIKE metacharacters in the key, so a _/% in a field name is matched literally instead of acting as a wildcard (which previously leaked sibling keys — see #69). The [%] array token is left as a deliberate wildcard (match any index).