how to ignore internal 404 response and only activate a irule on a full page 404 response ( such as css, image etc. )

You could go either I think, but you'd more or less have to define all page objects or all non-page objects, or at least object extensions. Here's an example that defines non-page objects from a data group.

when HTTP_REQUEST {
    if { [class match [string tolower [HTTP::uri]] ends_with objectlist] } {
        set do_not_err 1
    }
}
when HTTP_RESPONSE {
    if { [info exists do_not_err] } {
        unset do_not_err
        return
    } elseif { ( [HTTP::status] starts_with "40" ) or ( [HTTP::status] starts_with  "50" ) } {
        HTTP::respond 200 content "Broken"
    }
}

where objectlist is a string-based data group that contains the lowercased extensions of known object types that you might encounter on a page. Example.

jpg
gif
png
js
css