## Summary
Fixes#15534 — `update_document_name_only()` crashes with
`AttributeError` when `File2Document` exists but the linked `File` row
was deleted.
`update_document_name_only()` in `document_api_service.py` called
`FileService.get_by_id()` when a `File2Document` row existed, then
accessed `file.id` without checking the lookup result. An orphan
`File2Document` link (file deleted, mapping left behind) caused document
rename via `PATCH /api/v1/datasets/{dataset_id}/documents/{document_id}`
to return HTTP 500.
This PR mirrors guards used in `file2document_api.py` and
`file_api_service.py`: skip the optional file rename when the file is
missing, and still update the document record and search index.
## Changes
- `api/apps/services/document_api_service.py` — check `e and file`
before `FileService.update_by_id`
- `test/unit_test/api/apps/services/test_update_document_name_only.py` —
regression tests (orphan link + happy path)
## Test plan
- [x] `pytest
test/unit_test/api/apps/services/test_update_document_name_only.py -v`
- [ ] Manual: PATCH document `name` when `File2Document` points to a
non-existent `file_id` → 200, document/index renamed, no 500
