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.

test/spec/Main.hs

@@ -126,7 +126,6 @@ main = do
 
       extraSearchPathApp   = appDbs testCfgExtraSearchPath
       unicodeApp           = appDbs testUnicodeCfg
-      nonexistentSchemaApp = appDbs testNonexistentSchemaCfg
       multipleSchemaApp    = appDbs testMultipleSchemaCfg
       ignorePrivOpenApi    = appDbs testIgnorePrivOpenApiCfg
 
@@ -221,10 +220,6 @@ main = do
     parallel $ before asymJwkSetApp $
       describe "Feature.Auth.AsymmetricJwtSpec" Feature.Auth.AsymmetricJwtSpec.spec
 
-    -- this test runs with a nonexistent db-schema
-    parallel $ before nonexistentSchemaApp $
-      describe "Feature.Query.NonExistentSchemaErrorSpec" Feature.Query.ErrorSpec.nonExistentSchema
-
     -- this test runs with an extra search path
     parallel $ before extraSearchPathApp $ do
       describe "Feature.ExtraSearchPathSpec" Feature.ExtraSearchPathSpec.spec