fix(disputed): add admin role + contact info in discussion modal

- request-discussion endpoint: add 'admin' to allowed roles
- Return rejector_email and rejector_role in the response
- Modal success state shows contact card with username, role, email link
  so the approving lead can immediately reach out to the rejecting lead

frontend/src/api/tests.ts

@@ -313,6 +313,8 @@ export async function requestDiscussion(testId: string): Promise<{
   status: string;
   message: string;
   rejector_username: string;
+  rejector_email: string | null;
+  rejector_role: string;
 }> {
   const { data } = await client.post(`/tests/${testId}/request-discussion`);
   return data;