This patch enforces that vector indexes can only be created on keyspaces
that use tablets. During index validation, `check_uses_tablets()` verifies
the base keyspace configuration and rejects creation otherwise.
To support this, the `custom_index::validate()` API now receives a
`const data_dictionary::database&` parameter, allowing index
implementations to access keyspace-level settings during DDL validation.
Fixes https://scylladb.atlassian.net/browse/VECTOR-322Closesscylladb/scylladb#26786
(cherry picked from commit 68c7236acb)
Closesscylladb/scylladb#27272
2e0c41b32b