Function attach_protocol 

pub fn attach_protocol(
    request_xml: &str,
    response_xml: &str,
) -> Result<String, FiscalError>

Attach the SEFAZ authorization protocol to a signed NFe XML, producing the <nfeProc> wrapper required for storage and DANFE.

The function extracts the <NFe> from request_xml and the matching <protNFe> from response_xml, validates the protocol status, and joins them into a single <nfeProc> document.

If the response contains multiple <protNFe> nodes (batch response), the function attempts to match by digest value and access key. When no exact match is found it falls back to the first available <protNFe>.

Errors

Returns FiscalError::XmlParsing if:

• Either input is empty
• The <NFe> tag is missing from request_xml
• No <protNFe> can be found in response_xml

Returns FiscalError::SefazRejection if the protocol status code is not in VALID_PROTOCOL_STATUSES.