Users ought to have possibility to create the local index for Vector Search
based only on a part of the partition key. This commits provides this by
removing requirements of 'full partition key only' for custom local index.
The commit updates docs to explain that local vector index can use only a part
of the partition key.
The commit implements cqlpy test to check fixed functionality.
Fixes: SCYLLADB-953
Needs to be backported to 2026.1 as it is a fix for local vector indexes.
Closesscylladb/scylladb#28931