Typed handle for LibreOfficeSession.load_workbook / close_workbook #77
Description
Background
LibreOfficeSession.load_workbook() currently returns a raw dict token and LibreOfficeSession.close_workbook() is a no-op. This was flagged during the review of PR #76 (#76) and deferred as out of scope for that PR.
As noted by the reviewer, the current public session contract is easy to misuse and provides mypy with almost no useful type information.
Suggested improvement
Replace the raw dict return with a proper typed handle (e.g., a frozen dataclass LibreOfficeWorkbookHandle), and implement meaningful cleanup logic in close_workbook() once the subprocess lifecycle is formalised.
Reference comment: #76 (comment)
Requested by: @harumiWeb