Don't show stock for on-demand inventory items

[mkllnk]

What? Why?

• Closes #???

It seems that we have a bug around subscriptions and negative stock levels. It has not been filed on Github yet but is discussed on Slack:

• https://openfoodnetwork.slack.com/archives/CDLKH9MM0/p1734668781966059

While the details are not all confirmed yet, one confusing part is that on-demand products show negative stock levels on the inventory page. The UI was assuming that on-demand products don't have a stock level set. But we are tracking stock levels since Track (negative) stock for on-demand products and overrides #12726.

I'm a bit hesitant to present this fix because it could make investigation of the actual bug more difficult by hiding the stock level. Ideally, we would solve the subscription bug first but we don't have a confirmed description yet. So we may wait merging this until we started work on the subscription issue.

What should we test?

• Visit the inventory page.
• Have variants with overriding stock management.
• Have on demand and stock controlled variants.
• Stock controlled variants should show a stock level.
• On-demand variants should not show a stock level.
• Place an order for an on-demand variant so that its stock level becomes negative.
• You can check the stock level on the products page by unticking on-demand without saving.
• On-demand variants should still not show a stock level on the inventory page.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• User facing changes
• API changes (V0, V1, DFC or Webhook)
• Technical changes only
• Feature toggled

The title of the pull request will be included in the release notes.

Dependencies

Documentation updates

[dacook]

Good fix, but waiting to merge sounds like a good idea.

[rioug]

Looks good 👍

[filipefurtad0]

Hey @mkllnk,

I've set up 4 variants on the inventory page:
i) on demand
ii) on demand, overridden
iii) stock controlled, overridden
iv) stock controlled

Before this PR, these look like this:

As expected by this PR, after staging, the variant on position i) does not display stock:

Checking the stock by un-ticking the on demand checkbox temporarily displays the stock:

So, it all seems to work as expected. From the discussion above, I'm unsure whether it is a good idea to merge this now.
Can you confirm @mkllnk ?

[mkllnk]

The other issue has still not been filed. So maybe it's not actually a problem at the moment. After this pull request, we can still check stock on the products page by unchecking the on-demand box as done when testing here. So it's not a blocker. I'll merge.