cog_open() connected with a bare dbConnect() and set no resource pragmas, so
DuckDB claimed every visible core. Right for one interactive session on a
dedicated machine; wrong for a server, where cog-api runs two replicas on an
8-core host budgeted 4 and each replica independently claims all 8.
USCOGDATA_DUCKDB_THREADS and USCOGDATA_DUCKDB_MEMORY_LIMIT now resolve through
.cfg() -- inheriting the env var > option > default precedence USCOGDATA_URL
already had -- and are applied as pragmas when the connection is created.
Unset issues NO pragma, so an unconfigured session is byte-identical to before.
That negative property is asserted directly against a connection opened the
pre-change way rather than against a hardcoded core count.
.cfg() returns an env var as character, so both resolvers coerce and validate
rather than trusting the type: sprintf("SET threads TO %d", "4") would
otherwise abort inside the connection path with an error naming the pragma
instead of the setting the operator got wrong.
Replaces cog-api's getFromNamespace(".ensure_session", "uscogdata") workaround,
which depended on a private name and on the session already being open.
