Reads the columns of one table, view or materialized view of a data warehouse source (Snowflake, BigQuery or Databricks) from the source itself, through its metadata API: BigQuery's table metadata, Snowflake's DESCRIBE TABLE without a warehouse, or Delta Sharing table metadata. Only tables the source's own catalog covers are read: a dataset its BigQuery catalog lists, its Databricks share, or its Snowflake account outside Bobsled's admin database. A 404 means no such table exists there under these exact names, or Bobsled cannot read it.