Regression: cleanup_namespaces removes also xs + xsi prefixes

[vladimir-mencl-eresearch]

The cleanup_namespaces call added in #325 considers only namespaces used in XML Element and Attribute names, but not in attribute values.

The xs namespaces is used only in values of of xsi:type attributes, such as in:

        <saml:Attribute Name="http://macedir.org/entity-category" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri">
          <saml:AttributeValue xsi:type="xs:string">https://refeds.org/category/code-of-conduct/v2</saml:AttributeValue>
        </saml:Attribute>

The LXML cleanup_namespaces considers it unused and removes, resulting into metadata that fails schema validation:

xmllint --noout --schema /opt/mdtool/app/web/schemas/schema.xsd /var/www/html/metadata/tuakiri-dev-metadata-signed.xml
/var/www/html/metadata/tuakiri-dev-metadata-signed.xml:1095: element AttributeValue: Schemas validity error : Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue', attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName value 'xs:string' has no corresponding namespace declaration in scope.
/var/www/html/metadata/tuakiri-dev-metadata-signed.xml:1096: element AttributeValue: Schemas validity error : Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue', attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName value 'xs:string' has no corresponding namespace declaration in scope.
/var/www/html/metadata/tuakiri-dev-metadata-signed.xml:1099: element AttributeValue: Schemas validity error : Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue', attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName value 'xs:string' has no corresponding namespace declaration in scope.
/var/www/html/metadata/tuakiri-dev-metadata-signed.xml:1100: element AttributeValue: Schemas validity error : Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue', attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName value 'xs:string' has no corresponding namespace declaration in scope.
/var/www/html/metadata/tuakiri-dev-metadata-signed.xml fails to validate

And also fails with Shibboleth SP using MetadataProvider with validate="true":

2026-09-03 11:06:07 ERROR XMLTooling.ParserPool : fatal error on line 6018, column 53, message: prefix 'xs' can not be r
esolved to namespace URI
2026-09-03 11:06:07 ERROR OpenSAML.MetadataProvider.XML : error while loading resource (/var/cache/shibboleth/metadata.t
uakiri-test.xml): XML error(s) during parsing, check log for specifics
2026-09-03 11:06:07 CRIT OpenSAML.MetadataProvider.Chaining : failure initializing MetadataProvider: XML error(s) during
 parsing, check log for specifics

This can be solved with lxml.etree.cleanup_namespaces keep_ns_prefixes parameter.