Admin server /ready endpoint is incorrect on slow schema cache loads

[steve-chavez]

Problem

The schema cache doesn't finish loading:

# nix-shell
$ PGRST_ADMIN_SERVER_PORT=3001  PGRST_DB_SCHEMAS="apflora" PGRST_DB_POOL=3 postgrest-with-postgresql-16 -f test/io/big_schema.sql postgrest-run

...
15/Mar/2024:09:47:13 -0500: Listening for notifications on the pgrst channel
15/Mar/2024:09:47:13 -0500: Config reloaded
15/Mar/2024:09:47:13 -0500: Schema cache queried in 27.1 milliseconds

# still no "Schema cache loaded"

And yet the ready endpoint replies successfully:

$ curl localhost:3001/ready -i

HTTP/1.1 200 OK

This is due to

postgrest/src/PostgREST/Admin.hs

Line 41 in 4b289b1

isSchemaCacheLoaded <- isJust <$> AppState.getSchemaCache appState

That checks for isJust, which IS the state of the schema cache while loading but it's not yet filled (a traceShowId confirms this).