fix(neighbors): center node properties not hydrated in neighbors query response
## Summary
When running a neighbors query, the center node appears in the `nodes` array with only `type` and `id`. The `columns` specified on the center node's `NodeSelector` are not hydrated.
## Reproduction
```bash
curl -s "https://staging.gitlab.com/api/v4/orbit/query" \
-H "PRIVATE-TOKEN: $TOKEN" \
-H "Content-Type: application/json" \
-d '{"query":{"query_type":"neighbors","node":{"id":"u","entity":"User","node_ids":[64248],"columns":["id","username","name"]},"neighbors":{"node":"u","direction":"outgoing"},"limit":5},"response_format":"raw"}'
```
## Expected
Center node includes requested columns:
```json
{"type": "User", "id": 64248, "username": "stanhu", "name": "Stan Hu"}
```
## Actual
Center node has only type and id:
```json
{"type": "User", "id": 64248}
```
Neighbor nodes are hydrated with their default columns, but the center node is not.
Found during GOON format design research (staging validation).
issue