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/wizards/import_wizard.py

@@ -60,6 +60,22 @@ class FusionBillingImportWizard(models.TransientModel):
             "target": "new",
         }
 
+    def action_test_connection(self):
+        """Read-only connectivity + schema check: connect, read the source tables, and
+        report row counts WITHOUT importing anything. The safe first step before a
+        dry-run — surfaces a bad DSN, no network route, or a schema drift up front."""
+        self.ensure_one()
+        data = self._read_nexacloud_rows()
+        msg = "Connected. Read %s user(s), %s plan(s), %s subscription(s)." % (
+            len(data.get("users", [])), len(data.get("plans", [])),
+            len(data.get("subscriptions", [])))
+        return {
+            "type": "ir.actions.client",
+            "tag": "display_notification",
+            "params": {"title": "NexaCloud connection OK", "message": msg,
+                       "type": "success", "sticky": False},
+        }
+
     # ----- read side (the ONLY code that touches NexaCloud) ------------------
     def _read_nexacloud_rows(self):
         """Open a READ-ONLY psycopg2 connection to the nexacloud Postgres (DSN in