feat: Fail schema cache lookup with invalid db-schemas config

Previously, we'd silently report "200 OK" on the root endpoint, but
would never return any endpoints from the schema cache.

Now the schema cache query fails because of the ::regnamespace cast.

CHANGELOG.md

@@ -25,6 +25,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
  - #2052, Dropped support for PostgreSQL 11 - @wolfgangwalther
  - #3508, PostgREST now fails to start when `server-port` and `admin-server-port` config options are the same - @develop7
  - #3607, PostgREST now fails to start when the JWT secret is less than 32 characters long - @laurenceisla
+ - #3644, Fail schema cache lookup with invalid db-schemas config - @wolfgangwalther
+   - Previously, this would silently return 200 - OK on the root endpoint, but don't provide any usable endpoints.
 
 ## [12.2.1] - 2024-06-27