fix: Use get_index() instead of list_indexes() in has_collection() to… (#19238)

* fix: Use get_index() instead of list_indexes() in has_collection() to handle pagination

Fixes #19233

  Replace list_indexes() pagination scan with direct get_index() lookup
  in has_collection() method. The previous implementation only checked
  the first ~1,000 indexes due to unhandled pagination, causing RAG
  queries to fail for indexes beyond the first page.

  Benefits:
  - Handles buckets with any number of indexes (no pagination needed)
  - ~8x faster (0.19s vs 1.53s in testing)
  - Proper exception handling for ResourceNotFoundException
  - Scales to millions of indexes

* Update s3vector.py

Unneeded exception handling removed to match original OWUI code

backend/open_webui/retrieval/vector/dbs/s3vector.py

@@ -116,17 +116,19 @@ class S3VectorClient(VectorDBBase):
         return filtered_metadata
 
     def has_collection(self, collection_name: str) -> bool:
-        """
-        Check if a vector index (collection) exists in the S3 vector bucket.
-        """
-
-        try:
-            response = self.client.list_indexes(vectorBucketName=self.bucket_name)
-            indexes = response.get("indexes", [])
-            return any(idx.get("indexName") == collection_name for idx in indexes)
-        except Exception as e:
-            log.error(f"Error listing indexes: {e}")
-            return False
+          """
+          Check if a vector index exists using direct lookup.
+          This avoids pagination issues with list_indexes() and is significantly faster.
+          """
+          try:
+              self.client.get_index(
+                  vectorBucketName=self.bucket_name,
+                  indexName=collection_name
+              )
+              return True
+          except Exception as e:
+              log.error(f"Error checking if index '{collection_name}' exists: {e}")
+              return False
 
     def delete_collection(self, collection_name: str) -> None:
         """