Add ValidateCreateSchema analyzer rule that checks if the current
database has a valid root before allowing CREATE SCHEMA to proceed.
This ensures PostgreSQL-compliant behavior where schemas must be
created within an existing database.

Adds Go integration test and bats end-to-end tests to verify the
following:
- Connection to non-existent database fails with error
- CREATE SCHEMA succeeds on valid database

Refs: dolthub#1863