[FTS] : Invalid knn operator fails with grpc_client: query got status code: 500, resp: &bleve.SearchResult{Status:(*bleve.SearchStatus)(0x400007b0c0), Request:(*bleve.SearchRequest)(0x4004f19180)

Steps to repro

1. Created a 4 node cluster with 2 fts node, 1 kv and 1 query
2. Created a couchstore bucket and loaded some docs
3. Created a search index with with 2 mapping child fields
   1. field - content, type - text, searchable as - content, analyser - en
   2. field - vector, type - vector, searchable as - vector, dimension - 384, similarity metric - dot_product
4. Ran a search query on the index with "knn_operator":"AND"
5. The search query fails 
6. Ran a search query on the index with "knn_operator":"and" and the query works

Search query is attached here - 
https://cb-engineering.s3.amazonaws.com/MB-60076/search_query.json
 
The expected behaviour is to give an error message displaying that the operator is not valid