[Outlook] outlook_cloud: Graph attachments (mail, calendar, task)

## Parent

Part of #4264
Blocked by: Graph tasks sync

## Summary

Implement Graph file attachment download for mail, calendar, and task parents with parity to current attachment rules.

## Scope

- Attachment metadata + `$value` download
- Preserve: file attachments only, extension/size filters, text extraction service hook
- Golden tests for Mail / Calendar / Task attachment types

## Acceptance criteria

- [ ] Three attachment doc types match golden contract
- [ ] Item attachments skipped; filters preserved
- [ ] Unit tests green

## Out of scope

- DLS, incremental

Start by reading the current attachment rules and the Graph tasks sync work referenced in the issue. Then run the golden and unit tests for Mail, Calendar, and Task attachment types; done means file attachments download through metadata and `$value`, filters and the extraction hook remain intact, and item attachments are skipped.

Written by the indexing model from the issue text.