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/api/services.ts

@@ -2,6 +2,7 @@ import { del, get, post, put } from "./shared";
 import type {
 	ServiceInstance,
 	ServiceInstanceInput,
+	ServiceTestResult,
 	ServiceTypeInfo,
 } from "../types";
 
@@ -36,3 +37,9 @@ export async function deleteServiceInstance(
 ): Promise<{ status: string }> {
 	return del<{ status: string }>(`/api/services/instances/${serviceId}`);
 }
+
+export async function testServiceInstance(
+	input: ServiceInstanceInput,
+): Promise<ServiceTestResult> {
+	return post<ServiceTestResult>("/api/services/test", input);
+}