i18next-http-middleware: MissingKeyHandler does not reject keys whose segments contain prototype-polluting names

Impact i18next-http-middleware ≤ 3.9.6's missingKeyHandler blocked the literal request-body keys proto, constructor, and prototype added in 3.9.3, see GHSA-5fgg-jcpf-8jjw, but did not reject dotted variants such as "proto.polluted". Downstream backends that split the missing-key string on a...