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/views/import_wizard_views.xml

@@ -19,6 +19,8 @@
                     <field name="result_summary" nolabel="1" widget="text"/>
                 </group>
                 <footer>
+                    <button name="action_test_connection" type="object"
+                            string="Test Connection" class="btn-secondary"/>
                     <button name="action_run_import" type="object" string="Run Import"
                             class="btn-primary"/>
                     <button string="Close" class="btn-secondary" special="cancel"/>