Replace need for KnnVectorValues.copy() with a dictionary interface

### Description

The idea is to get rid of `copy()` since it does more than is needed. What's needed is an independent vector accessor over the same values source. Adrien had suggested a dictionary() interface that would take over the random-access API from the values source (or in addition to it?)
See discussion in https://github.com/apache/lucene/pull/13779 for more details.

Start by reading the discussion in Apache Lucene pull request 13779, then trace the uses of KnnVectorValues.copy() that motivate this issue. Done means agreeing on and implementing an independent vector accessor or dictionary() interface over the same values source, with the affected behavior covered by the relevant tests.

Written by the indexing model from the issue text.