### What problem does this PR solve?
Partially addresses #14362 by adding deleted-file sync support for the
Dropbox data source.
Dropbox previously did not provide the slim current-file snapshot
required by stale document reconciliation, and its sync runner returned
only document batches. As a result, enabling deleted-file sync could not
remove local documents that had been deleted from Dropbox.
This PR:
- Adds `retrieve_all_slim_docs_perm_sync()` to `DropboxConnector`.
- Reuses Dropbox metadata traversal to collect current remote file IDs
without downloading file contents.
- Wires incremental Dropbox sync to return `(document_generator,
file_list)` when `sync_deleted_files` is enabled.
- Enables the deleted-file sync toggle for Dropbox in the data source
settings UI.
- Adds regression coverage for slim snapshots, nested folders, paginated
listings, duplicate filenames, and full reindex behavior.
Tests:
- `uv run pytest test/unit_test/common/test_dropbox_connector.py -q`
- `uv run pytest test/unit_test/rag/test_sync_data_source.py -q`
- `uv run pytest test/unit_test/common/test_dropbox_connector.py
test/unit_test/rag/test_sync_data_source.py -q`
- `uv run ruff check common/data_source/dropbox_connector.py
rag/svr/sync_data_source.py
test/unit_test/common/test_dropbox_connector.py
test/unit_test/rag/test_sync_data_source.py`
- `./node_modules/.bin/eslint
src/pages/user-setting/data-source/constant/index.tsx`
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
When there are multiple files with the same name the file would just
duplicate, making it hard to distinguish between the different files.
Now if there are multiple files with the same name, they will be named
after their folder path in the storage unit.
This was done for the webdav connector and with this PR also for Notion,
Confluence and S3 Storage.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
Contribution by RAGcon GmbH, visit us [here](https://www.ragcon.ai/)
