Fix customerId always returning null in Cart API responses

[axel-paillaud]

Questions	Answers
Branch?	9.1.x
Description?	Fix missing customerId in GetCartForOrderCreation query result
Type?	bug fix
Category?	CO
BC breaks?	no
Deprecations?	no
How to test?	Call the Cart-POST create endpoint with a customer ID. The response body now correctly returns the customer ID instead of null.
UI Tests	https://github.com/axel-paillaud/ga.tests.ui.pr/actions/runs/25147314600
Fixed issue or discussion?	Fix Admin API endpoint Cart response customer id = null (in progress)
Related PRs	If theme, autoupgrade or other module change is needed to make this change work, provide a link to related PRs here.
Sponsor company	axel-paillaud

The Cart API resource exposes a customerId field, but it was always null in responses
(GET /carts/{cartId} and POST /carts).

The root cause was in CartForOrderCreation, the query result returned by GetCartForOrderCreation:
it did not include the customer ID at all. Since API Platform maps query result getters to the
resource properties by name, the field was present in the response but never populated.

The fix adds customerId to CartForOrderCreation (property, constructor parameter, and getter),
and updates GetCartForOrderCreationHandler to pass $cart->id_customer when building the result

Cart endpoints available here

[ps-jarvis]

Hi, thanks for this contribution!

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[Hlavtox]

BC break, you are adding required parameters