Investigate handling DuckDB schemas safely

[begelundmuller]

Following up on this thread: https://rilldata.slack.com/archives/CTCJ58H3M/p1736339712522279?thread_ts=1736192001.051219&cid=CTCJ58H3M.

We actually also have some code that captures the current database/schema in the model reconciler and other places. I wonder if that can lead to some inconsistencies if the external table storage version gets captured there.

I'm wondering if we should just always return "main" or "" in the information schema listings to give the appearance of it being one schema, and then audit the code to avoid ever making fully qualified table names (i.e. database.schema.table) in our code. If a user has a fully qualified table name in their model code, it's acceptable to error.