feat(service-credential-tester): slice 2 — Test button + gating (shared ServiceTestPanel)

Presentational ServiceTestPanel (props-driven, no internal hooks) wired into
both CreateServiceDialog (ServicesPage.tsx) and ServiceConfigEditor
(Settings.tsx). Parent owns testResult + saveAnyway state; store-previous
pattern resets on input change (avoids setState-in-effect). Create/Save
button gated on testPassed || saveAnyway. 7 panel tests (button states,
success/failure pills, checkbox toggle). All gates: 158 vitest, build exit 0,
lint 0 errors, 362 backend pytest (regression).

frontend/src/hooks/useServices.ts

@@ -4,6 +4,7 @@ import {
 	deleteServiceInstance,
 	fetchServiceInstances,
 	fetchServiceTypes,
+	testServiceInstance,
 	updateServiceInstance,
 } from "../api/services";
 import type { ServiceInstanceInput } from "../types";
@@ -45,3 +46,9 @@ export function useDeleteServiceInstance() {
 		},
 	});
 }
+
+export function useTestServiceInstance() {
+	return useMutation({
+		mutationFn: (input: ServiceInstanceInput) => testServiceInstance(input),
+	});
+}