Avoid re-validating UTF-8 in `FromUtf8Error::into_utf8_lossy` Part of the unstable feature `string_from_utf8_lossy_owned` - #129436 Refactor `FromUtf8Error::into_utf8_lossy` to copy valid UTF-8 bytes into the buffer, avoiding double validation of bytes. Add tests that mirror the `String::from_utf8_lossy` tests.