Fix: Use self.dataStore.indexExist in all_tags method of Dealer (#5108)

### What problem does this PR solve?

This PR fixes an AttributeError in the all_tags method of the Dealer
class. Previously, the method incorrectly called
self.docStoreConn.indexExist instead of self.dataStore.indexExist. Since
self.docStoreConn was never set (and self.dataStore is already
initialized in init), this resulted in an error when attempting to check
if the index exists. This change ensures that the proper connector is
used for the index existence check, thereby resolving the issue._

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/nlp/search.py

@@ -449,7 +449,7 @@ class Dealer:
         return res
 
     def all_tags(self, tenant_id: str, kb_ids: list[str], S=1000):
-        if not self.docStoreConn.indexExist(index_name(tenant_id), kb_ids[0]):
+        if not self.dataStore.indexExist(index_name(tenant_id), kb_ids[0]):
             return []
         res = self.dataStore.search([], [], {}, [], OrderByExpr(), 0, 0, index_name(tenant_id), kb_ids, ["tag_kwd"])
         return self.dataStore.getAggregation(res, "tag_kwd")