Validate and re-enable OneDrive File Sync

## Summary
Validate the existing OneDrive connector end to end, then decide whether to re-enable it in Admin Settings source visibility.

## User Value
Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

## Proposed Behavior
Confirm selected-path browse, Graph application permissions, global File Sync identity, personal-only scope, remote delete behavior, and sync-run observability.

## Acceptance Criteria
- [ ] The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
- [ ] Permission, configuration, and existing-feature interactions are accounted for.
- [ ] Tests, documentation, or validation notes are added as appropriate.

## Notes
- Source backlog entry: Revalidate And Re-enable OneDrive Sync
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension / Verify
- Current anchors: `docs/explanation/features/ONEDRIVE_FILE_SYNC.md`, `docs/explanation/features/FILE_SYNC_SOURCE_WORKFLOW.md`, `application/single_app/functions_file_sync.py`
- Proposed roadmap priority: P2
- Proposed roadmap size: M