[Outlook] outlook_cloud: DLS + access control sync

## Parent

Part of #4264
Blocked by: Graph attachments

## Summary

Port DLS / access-control sync to `outlook_cloud` with mailbox-level ACL decoration parity.

## Scope

- Port `_prefix_email` / identity doc generation from Outlook `utils.py`
- Mailbox-level ACL decoration parity (`_mailbox_access_control`)
- Reuse DLS test patterns from `test_outlook.py`

## Acceptance criteria

- [ ] Identity docs + content ACL decoration match dialect of legacy Outlook
- [ ] Email casing / reachability tests ported and green
- [ ] DLS toggle behaves like legacy

## Out of scope

- Incremental sync

Start with `outlook_cloud` and compare the legacy Outlook `utils.py` implementation of `_prefix_email` and identity document generation. Review `_mailbox_access_control` and the DLS test patterns in `test_outlook.py`; port the tests and use them to check ACL decoration parity, email casing and reachability, and DLS toggle behavior. Incremental sync is out of scope.

Written by the indexing model from the issue text.