[e2e] ensure user registration runs in a fresh session (#31164)

* [e2e] ensure user registration runs in a fresh session

* [e2e] align session key in Server Pro and SaaS

Co-authored-by: Antoine Clausse <antoine.clausse@overleaf.com>

---------

Co-authored-by: Antoine Clausse <antoine.clausse@overleaf.com>
GitOrigin-RevId: cc322217dc5a631c2795f477edaa90da8b83959e

server-ce/test/helpers/login.ts

@@ -57,7 +57,7 @@ export function ensureUserExists({
 
 export function login(username: string, password = DEFAULT_PASSWORD) {
   cy.session(
-    [username, password],
+    ['via-login', username, password],
     () => {
       cy.visit('/login')
       cy.get('input[name="email"]').type(username)