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/SpecHelper.hs

@@ -227,9 +227,6 @@ testCfgAsymJWKSet =
   , configJWKS = rightToMaybe $ parseSecret secret
   }
 
-testNonexistentSchemaCfg :: AppConfig
-testNonexistentSchemaCfg = baseCfg { configDbSchemas = fromList ["nonexistent"] }
-
 testCfgExtraSearchPath :: AppConfig
 testCfgExtraSearchPath = baseCfg { configDbExtraSearchPath = ["public", "extensions", "EXTRA \"@/\\#~_-"] }