Fix Document::saveAs with QSaveFile

[lite-tx]

Summary

• preserve QSaveFile ownership while allowing QZipWriter to finalize the ZIP archive
• keep the existing close behavior for other QIODevice implementations
• document that callers must commit a QSaveFile after Document::saveAs()

QZipWriter::close() must be called to write the ZIP central directory, but it also unconditionally closes its device. Closing a QSaveFile aborts the process because it must be finalized with commit(). The new proxy forwards I/O and seek operations while intercepting only close(), so the caller can commit the completed workbook atomically.

Verification

• reproduced the original QSaveFile::close called fatal exit on current master
• verified both pre-opened and unopened QSaveFile instances can save, remain open, commit, and be read back
• verified existing QFile and QBuffer close behavior and workbook read-back
• independently validated generated archives with Python zipfile -t
• built and ran the regression with Qt 5.15.2/MSVC in Debug, Release static, and Release shared configurations
• built and ran the regression with Qt 6.6.0/MSVC in Release static configuration
• built and ran TestExcel through qmake/nmake

Fixes #443

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud