Function parse 

pub fn parse(input: &str) -> Result<OfxDocument, OfxError>

Parses a complete OFX document from a string.

This is the primary entry point for the library. It accepts the full content of an OFX file (header + XML body) and returns a structured document.

Errors

Returns OfxError if the header is missing or malformed, the XML is invalid, or required OFX elements are missing.