fix: short-circuit list_schemas to skip ~500x storm in before_run

[0xRobin]

Summary

Every dbt run against prod issues ~500 identical select schema_name from hive.INFORMATION_SCHEMA.schemata queries during before_run (~5 min wall time on hourly). Root cause is a Set[BaseRelation] dedup bug in dbt-adapters — __hash__ is render-based but __eq__ is to_dict-based, and .include(schema=False, identifier=False) doesn't clear the underlying path fields, so the set keeps every entry instead of one per unique database.

Upstream fix: dbt-labs/dbt-adapters#1930. This PR is a project-side workaround until that lands and ships.

Change

• dbt_macros/dune/no-relation-listing.sql: list_schemas now always returns []. dbt-core falls through to dispatching create_schema for each unique (db, schema) string tuple (that dedup is native string sets, not BaseRelation, and works correctly).
• dbt_macros/dune/schema.sql: add IF NOT EXISTS to the hive branch of trino__create_schema so the fallback dispatch is a metastore-cheap no-op (single getDatabase Thrift call) on already-existing schemas instead of a full information_schema.schemata scan.

Expected effect

Before:

~525 identical `select schema_name from hive.INFORMATION_SCHEMA.schemata`
~5 min wall time on `before_run`

After:

0 `list_schemas` queries
~50 `CREATE SCHEMA IF NOT EXISTS` calls (one per unique (db, schema))
~3 s on `before_run`

Pulled from production debug.log (dbt_cloud_run_id=70471878875888).

Risk

list_schemas returning [] would lie to any other caller that depends on knowing which schemas exist. Spellbook has no such caller (grep against sources/, dbt_subprojects/, dbt_macros/ is empty). The only Python caller in dbt-core is RunTask.create_schemas, which is precisely the path we want to short-circuit.

CREATE SCHEMA IF NOT EXISTS ... WITH (location=...) is valid Trino syntax (the WITH clause is ignored for existing schemas).

Once dbt-labs/dbt-adapters#1930 ships in our pinned dbt-adapters version, this macro should be reverted to the dispatched form.

[cursor]

PR Summary

Medium Risk
Changes dbt macro behavior to always report no schemas and to always use CREATE SCHEMA IF NOT EXISTS, which could affect any workflows that rely on accurate schema discovery or strict create semantics; intended to reduce redundant metastore queries during runs.

Overview
Skips dbt’s expensive schema discovery during before_run by changing list_schemas to always return [], preventing hundreds of repeated information_schema.schemata queries.

Makes Trino schema creation idempotent by using CREATE SCHEMA IF NOT EXISTS ... WITH (location=...) for the non-dune branch, so the fallback create_schema path becomes a cheap no-op when schemas already exist.

^{Reviewed by Cursor Bugbot for commit 98da16e. Configure here.}