Fix Named Schema after(:all) to fully release plsql.connection

The Named Schema describe ended its after(:all) with
plsql.connection.logoff, which closes the underlying connection but
leaves plsql.connection pointing at the now-closed wrapper. Any
later example that attempted plsql.logoff if plsql.connection (for
example the after(:each) in "Connection with connect!") then
followed the stale reference into rollback on a dead OCI8 handle
and raised "OCI8 was already closed". Surfaced under random order
when "Named Schema" ran before "Connection with connect!" — the
"should not connect with wrong port number" example never replaced
the stale connection (its connect! is meant to fail), so its
after(:each) was the one that hit the dead handle.

Use plsql.logoff so the schema's setter clears @connection to nil.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

spec/plsql/schema_spec.rb

@@ -120,7 +120,7 @@
   end
 
   after(:all) do
-    plsql.connection.logoff
+    plsql.logoff
   end
 
   it "should find existing schema" do