[17.0][ADD] mail_attachment_office_preview: preview Office attachments via LibreOffice conversion

[alan196]

Replaces OCA/knowledge#635: moved here and renamed following the same naming criteria as #257 (@pedrobaeza's suggestion), since the module is about mail attachments and is not a derivative of attachment_preview.

Summary

New module that previews office documents (DOCX, XLSX, PPTX, DOC, XLS, PPT, ODT, ODS, ODP, ODG) in the browser — including chatter attachments — by converting them to PDF server-side with LibreOffice headless and rendering the result with Odoo's native PDF.js viewer.

Nothing leaves the server: no external viewer service (Microsoft Office Online, Google Docs) is involved, which makes it suitable for sensitive documents.

Context

• attachment_preview was never migrated to 17.0 and its ViewerJS-based approach only covers PDF + ODF anyway.
• [18.0][attachment_preview] Add Office format preview via LibreOffice conversion knowledge#603 adds Office-format preview to attachment_preview on 18.0 via LibreOffice conversion. This module brings the same conversion-endpoint logic to 17.0, but as a small standalone module: on 17.0 the native FileViewer already renders PDFs, so no ViewerJS or base module is needed.

How it works

• HTTP endpoint GET /mail_attachment_office_preview/to_pdf?model=&field=&id=&filename= converts the binary field content to PDF with libreoffice --headless --convert-to pdf (isolated UserInstallation profile per conversion, checksum-keyed disk cache under the filestore).
• A JS patch on the mail Attachment model marks office extensions as previewable (isPdf) and routes defaultSource through the conversion endpoint wrapped in the native PDF.js viewer. The Download button still returns the original file.
• Endpoint enforces: record read ACL, binary-field allow-list, extension allow-list, 50 MB size cap, 60 s conversion timeout. Returns 503 gracefully when LibreOffice is not installed.
• No new Python dependencies (stdlib subprocess/tempfile only).

Test plan

• Unit tests covering the conversion helper and every HTTP branch of the endpoint (LibreOffice is mocked, so tests pass on runners without it).
• Manually validated on Odoo.sh (LibreOffice 7.3.7): xlsx/docx/pptx attachments preview correctly in the chatter.

[pedrobaeza]

This module can't be tested, as the LibreOffice headless server is not included. Isn't this another way to preview them?

[alan196]

Added "external_dependencies": {"deb": ["libreoffice"]} to the manifest, as in report_py3o. Runboat installs the declared deb dependencies when the build starts, so LibreOffice should now be available and the preview testable there.

[alan196]

The red CI here is not caused by this PR: both failures are in mail_notify_employee_leave, whose tests only pass while the runner clock falls inside the employee's working schedule. Fixed in #260; once that is merged this PR should be green.

[pedrobaeza]

Tested on runboat and working. Please squash both commits into one.