Merge #5008

5008: Display vectors when no custom vectors where ever provided r=irevoire a=dureuill

# Pull Request

## Related issue
Fixes the issue reported on [Discord](https://discord.com/channels/1006923006964154428/1294653031958446080/1295336784896589967).

## What does this PR do?
- Normal behavior of Meilisearch is to hide `_vectors` even when `retrieveVectors: true` when there is an explicit list of displayed attributes that does not contain vectors
- However, this relied on the field id for the `_vectors` field to exist, which wasn't the case when no `_vectors` was manually provided to documents. This would often be the case for people using autoembedders such as the OpenAI integration.
- This PR fixes the behavior by looking for the `_vectors` string in the `displayedAttributes` when there is no `_vectors` fid.
- This PR also adds a test for this specific situation, that would fail before the PR, and pass after the PR


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch/src/search/mod.rs

@@ -1195,8 +1195,13 @@ impl<'a> HitMaker<'a> {
         let vectors_is_hidden = match (&displayed_ids, vectors_fid) {
             // displayed_ids is a wildcard, so `_vectors` can be displayed regardless of its fid
             (None, _) => false,
-            // displayed_ids is a finite list, and `_vectors` cannot be part of it because it is not an existing field
-            (Some(_), None) => true,
+            // vectors has no fid, so check its explicit name
+            (Some(_), None) => {
+                // unwrap as otherwise we'd go to the first one
+                let displayed_names = index.displayed_fields(rtxn)?.unwrap();
+                !displayed_names
+                    .contains(&milli::vector::parsed_vectors::RESERVED_VECTORS_FIELD_NAME)
+            }
             // displayed_ids is a finit list, so hide if `_vectors` is not part of it
             (Some(map), Some(vectors_fid)) => map.contains(&vectors_fid),
         };