[Atwix]Make the destination_cart_id argument optional in the mergeCarts mutation

[magento-engcom-team]

Make the destination*cart*id argument optional in the mergeCarts mutation

Why

PR
#30633

This came up in a discussion with [@sirugh](https://github.com/sirugh] from the [pwa-studio|https://github.com/magento/pwa-studio) team.

When a user logs in (creates a new token), one of the first things the UI needs to do is merge the current guest cart (if items are present) into the customer account's cart.

If destination*cart*id is required, this requires 3 round trips:

1. Call for Mutation.generateCustomerToken
2. Call for Query.cart to get customer cart ID
3. Call for Mutation.mergeCarts to merge guest cart ID into customer cart

Because a customer can only have a single cart, and this API only works for authenticated users, destination*cart*id is an unnecessary requirement here. If we make it optional, the login cart upgrade for UI can happen in a single request:

1. Mutations run serially, in-order. So `mergeCarts` will only execute
1. if `generateCustomerToken` succeeds
mutation LoginAndMergeCarts($email: String!, $password: String!, $guestCartID: String!) {
    generateCustomerToken(email: $email, password: $password) {
        token
    }
    mergeCarts(source*cart*id: $guestCartID) {
        ID
    }
} {code}
## Proposed Change

 
{code:java}
type Mutation {
-    mergeCarts(source*cart_id: String!, destination_cart*id: String!): Cart!
</ins>    mergeCarts(source*cart_id: String!, destination_cart*id: String): Cart!
} ```
<ins>**Approved Design**</ins>
 <https://github.com/magento/architecture/blob/mergeCarts-optional-destination/design-documents/graph-ql/coverage/mergeCarts-optional-destination.md> 
  

[magento-engcom-team]

The issue was exported from the internal JIRA. The link to the original JIRA issue: https://jira.corp.magento.com/browse/MC-37413

[m2-assistant]

Hi @magento-engcom-team. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento give me 2.4-develop instance - upcoming 2.4.x release

For more details, please, review the Magento Contributor Assistant documentation.

Please, add a comment to assign the issue: @magento I am working on this

---

• Join Magento Community Engineering Slack and ask your questions in #github channel.

⚠️ According to the Magento Contribution requirements, all issues must go through the Community Contributions Triage process. Community Contributions Triage is a public meeting.

🕙 You can find the schedule on the Magento Community Calendar page.

📞 The triage of issues happens in the queue order. If you want to speed up the delivery of your contribution, please join the Community Contributions Triage session to discuss the appropriate ticket.

🎥 You can find the recording of the previous Community Contributions Triage on the Magento Youtube Channel

✏️ Feel free to post questions/proposals/feedback related to the Community Contributions Triage process to the corresponding Slack Channel

[andrewbess]

@magento I am working on this