Order fails for product configured to not manage stock in custom website

[priyasbhat]

Preconditions (*)

1. Magento 2.3.0 installed and MSI enabled
2. Create new Website: testing_website
   
3. Create new Source - testing_source
4. Create new Stock - testing_stock add testing_source assine to testing_website*

Steps to reproduce (*)

1. Go to Catalog -> Products and create a product with Advanced Inventory, Manage Stock option set to "No". Product is listed in both default website and Test website. No sources are assigned since we do not want to manage stock for this product.
2. Go to Sales -> Orders and use the Create new order button to create an order. Select / create a customer and then select the Storeview for Test website to create order in.
3. Click Add Products and select product created in step 1 and click Add selected Product(s) to Order.

Expected result (*)

1. Product should be added successfully (screenshot attached)

Actual result (*)

1. You will see an error "This product is out of stock." displayed
2. On submitting order, the attached error is thrown.

Info is testing_website assigned to default stock it works fine

[priyasbhat]

The issue over here is that the check for stock is not being skipped when the product is configured to manage_stock = false. I have spoken to @maghamed regarding this and he acknowledged
it is an issue.

[larsroettig]

Documentation - how we can solve this Issue:

1. Change in Magento\InventoryIndexer we must change Index behaviour to:

Add all the products which have ManageStock = false, and along with that product which assigned to the website which uses this Stock.

Expected Behaviour
After Indexing entry is added to the added to correct inventory_stock_* DB Table

2. Change Implementation of: \Magento\Inventory\Model\ResourceModel\IsProductAssignedToStock::execute

This Implementation must ask the inventory_stock_* DB Table for is a Product (SKU) linked to
a Stock, not the inventory_source_stock_link DB Table.