27: IMAP custom FETCH listing classification skips literal boundaries, enabling response desynchronization

`is_custom_fetch_listing_match()` in `lib/imap.c` classifies any custom FETCH whose sequence set contains `:` or `,` as a "listing." For that classification, `imap_state_listsearch_resp()` skips literal detection entirely. The ping-pong line parser then scans the server-declared literal body as …

static bool is_custom_fetch_listing_match(const char *params)
{
  if(*params++ != ' ')
    return FALSE;
  while(ISDIGIT(*params)) {
    params++;
    if(*params == 0)
      return FALSE;
  }
  if(*params == ':')
    return TRUE;
  if(*params == ',')
    return TRUE;
  return FALSE;
}