feat(billing): importer Test Connection guard + operator runbook

Add action_test_connection — a read-only connectivity/schema check that
reports source row counts and imports nothing, the safe first step before
a dry-run. Wire a "Test Connection" button on the wizard. Document the
end-to-end run in the README: least-privilege read-only DB role SQL, the
fusion_billing.nexacloud_dsn system parameter (libpq DSN = NexaCloud's
URL minus +asyncpg), and the Test → dry-run → real-run flow. Refresh the
stale SCAFFOLD status. 53/53 green on odoo-trial.

fusion_centralize_billing/tests/test_importer.py

@@ -243,3 +243,9 @@ class TestImporterReadGuard(TransactionCase):
         wiz = self.env['fusion.billing.import.wizard'].sudo().create({'dry_run': True})
         with self.assertRaises(UserError):
             wiz._read_nexacloud_rows()
+
+    def test_test_connection_guards_missing_dsn(self):
+        self.env['ir.config_parameter'].sudo().set_param('fusion_billing.nexacloud_dsn', '')
+        wiz = self.env['fusion.billing.import.wizard'].sudo().create({'dry_run': True})
+        with self.assertRaises(UserError):
+            wiz.action_test_connection()