DuckDB repeated ensureSchema recreates canonical relation index #222

Problem

DuckDB's TeaQL schema adapter does not implement the authoritative TeaQLDatabase.indexExists lookup. A second UserContext.ensureSchema() for a model with a forward relation therefore attempts to create the same canonical relation index again and fails with DuckDB's Catalog Error: Index with name ... already exists.

This was independently reproduced in a one-parent/one-child runtime test on a fresh DuckDB file and in the generated task-graph test tracked by teaql/teaql-code-gen#276. No stale database file is involved.

Acceptance

• Look up secondary indexes in the active DuckDB database/schema and target table; metadata errors must fail closed.
• Two ensureSchema() calls leave one canonical relation index and both succeed.
• Run the DuckDB runtime suite and the generated task-graph test twice from a source-matched, isolated Maven cache.
• Do not suppress all duplicate-index errors or skip relation-index validation.