normalizePolicy static method
Normalizes a policy value: null resolves to def, while an empty
value or the none sentinel resolves to '', omitting the header.
Implementation
static String normalizePolicy(Object? value, String def) {
if (value == null) return def;
var s = value.toString().trim();
if (s.isEmpty || equalsIgnoreAsciiCase(s, 'none')) return '';
return s;
}