Introduce separate POST and PUT API for carts/mine/items

[ldc321]

Description

Currently, there is no way to set exact number of cart items. The POST request is adding posted quantity to existing quantity in the cart. The idea is to introduce ability to rewrite the items with PUT request

Steps to reproduce (*)

70096:ItemId already exists and qty=3
can{{t update cart items when set Maximum Qty Allowed in Shopping Cart
1.POST:http://xxxxxx.com/rest/V1/carts/mine/items
2.body:{"cartItem":{"sku":"AM-OL-I3T-CA-OS","qty":4,***"item_id":70096,_**"quoteId":"3xxx45","product_option":{"extension_attributes":{"custom_options":<],"configurable_item_options":[{"option_id":"93","option_value":"66"}],"bundle*options":[>}}}}
3.set Maximum Qty Allowed in Shopping Cart=6

If I do post, I get an error:The requested qty exceeds the maximum qty allowed in shopping cart
tips: If oldQty is 3, postQty is 2, final qty is 2, there is no problem!
if oldQty is 3, postQty is 4, I get an error:The requested qty exceeds the maximum qty allowed in shopping cart

Expected result (*)

Update: after review this issue expected result was updated #28162 (comment)

Desire state should be
POST }}carts/mine/items- add item(s). If called several times - increment Qty PUT carts/mine/items` - add item(s). If called several times - each time overwrite previously created item.

1. I want to change the qty of products in the cart 3 to 4

1. It should be overwrite, not increment

Actual result (*)

1. I get an error:The requested qty exceeds the maximum qty allowed in shopping cart

[m2-assistant]

Hi @ldc321. Thank you for your report.
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@ldc321 do you confirm that you were able to reproduce the issue on vanilla Magento instance following steps to reproduce?

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[ldc321]

My English is poor, I'm sorry

[m2-assistant]

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[magento-engcom-team]

✅ Confirmed by @shikhamis11
Thank you for verifying the issue. Based on the provided information internal tickets MC-34298 were created

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