Stock validation not working as expected with OrderLine custom fields

[andriinuts]

Describe the bug
Stock validation not working as expected when you are adding two of the same variants but with different custom field values.
For example, you have Variant A with tracked inventory, and in stock, you have only 1. You are calling addItemToOrder with Variant A and custom field value 1 and then calling addItemToOrder with the same Variant A but custom field value as 2. Now in order you have 2 order lines (the same variant with different custom fields, which are expected if it's available such as count of stock but not in this example when we have only 1 available stock).

To Reproduce
Steps to reproduce the behavior:

1. Create a custom field for OrderLine (for example customField with type string)
2. Create a Variant A with manageable stock and stock count 1
3. Create an order and add a Variant A with custom field value 1
4. Add one more time Variant A but now with custom field value 2
5. You successfully added more than the in-stock available items.

Expected behavior
Receiving an error that no possibility to add more of the Variant A to the order.

Environment (please complete the following information):

• @vendure/core version: 2.1.7
• Nodejs version: v20.10
• Database (mysql/postgres etc): any

Additional context
Also, you can fulfill this order (from steps to reproduce) and then you will have a negative stock value (-1)