WorkbookFactory throws ODFNotOfficeXmlFileException on .ods files —
Apache POI does not support ODF format at all.
Replace ODS reading with a direct content.xml parser using Java's
built-in ZipFile + DOM API (no new dependency). ODS is a ZIP archive;
the spreadsheet lives in content.xml as standard ODF XML.
Also refactors the import pipeline to decouple file reading from import
logic: both ODS and XLSX paths now produce List<List<String>> which is
processed by format-agnostic row logic. XLSX date cells are now
converted to ISO strings before processing.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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