feat(dropbox): support deleted-file sync (#14476)

### 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)

web/src/pages/user-setting/data-source/constant/index.tsx

@@ -73,6 +73,9 @@ export const DataSourceFeatureVisibilityMap = {
   [DataSourceKey.BOX]: {
     syncDeletedFiles: true,
   },
+  [DataSourceKey.DROPBOX]: {
+    syncDeletedFiles: true,
+  },
   [DataSourceKey.S3]: {
     syncDeletedFiles: true,
   },