Legacy: custom out of stock validation error fix

[garmashdev]

Description

Legacy mode: updated error response to the correct "out of stock" type for any custom exception which contain out of stock in its message

Fixes: update error response to correct "out of stock" type for any custom exception which contain out of stock in its message

#changelog Legacy: custom out of stock validation error fix

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please validate that you have tested your change in at least one of the following areas:

• Successfully tested locally (or docker image)
• Successfully tested on a staging or sandbox server
• Successfully tested on a merchant's staging server

For PR Reviewer

• Reviewed unit tests to make sure we are using real components rather than mocks as much as possible?
• For any major change (observer, new Bolt feature, core Magento interaction) we must add a feature switch, did you verify this?

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• New and existing unit tests pass locally with my changes.
• I have created or modified unit tests to sufficiently cover my changes.
• I have added my ticket link and provided a changelog message.