When index metadata was deserialized from system tables during schema
reload, target_parser::is_local() failed to recognize local vector
indexes. It only handled the non-vector JSON format {"pk": [...],
"ck": [...]}, but vector indexes serialize their targets as
{"pk": [...], "tc": "..."}. As a result, every local vector index was
incorrectly marked as global after a schema reload.
Fix this by introducing vector_index::is_local() that recognizes the
vector-specific target format, and dispatching to it from the schema
deserialization code based on the index class name. This keeps
target_parser as secondary-index-specific and follows the same dispatch
pattern already used for target serialization.
Also remove the now-unused has_vector_index_on_column() helper (its
callers were removed by #29407).
cf372ba87b