fix(core-flows): guest customer updates email to another guest account (#13037)

* fix(core-flows); guest customer updates email to another guest account

* wip: refactor test

* fix: refactor step, add testing

* fix: update test

* fix: string assertion in a test

packages/core/core-flows/src/cart/steps/find-or-create-customer.ts

@@ -39,12 +39,12 @@ interface StepCompensateInput {
 export const findOrCreateCustomerStepId = "find-or-create-customer"
 /**
  * This step finds or creates a customer based on the provided ID or email. It prioritizes finding the customer by ID, then by email.
- * 
+ *
  * The step creates a new customer either if:
- * 
+ *
  * - No customer is found with the provided ID and email;
  * - Or if it found the customer by ID but their email does not match the email in the input.
- * 
+ *
  * The step returns the details of the customer found or created, along with their email.
  */
 export const findOrCreateCustomerStep = createStep(
@@ -96,10 +96,14 @@ export const findOrCreateCustomerStep = createStep(
         })
       }
 
-      if (
-        !customer ||
-        (isDefined(data.email) && customer.email !== validatedEmail)
-      ) {
+      if (customer && customer.email !== validatedEmail) {
+        ;[customer] = await service.listCustomers({
+          email: validatedEmail,
+          has_account: false,
+        })
+      }
+
+      if (!customer) {
         customer = await service.createCustomers({ email: validatedEmail })
         customerWasCreated = true
       }