[Woo POS] Improve itemlist state representation and fix glitches

[joshheald]

Closes: #14350 Closes: #14251

Description

This PR removes uses of the items array and itemsPublisher from the ItemListViewModel. This is in preparation for architecture changes to move this state to the new Aggregated Model

The only remaining use of the items array is from the dashboard, where it's used for creating the order. We'll remove that in a future commit.

Instead, we rely on the ItemListState, which includes items when they should be shown.

As part of this work I've had to change the various pagination/loading/refresh behaviours to use the new source of truth for items, and along the way I've fixed some glitches

• Pull to refresh now leaves the simple products banner in place (arguably the items should remain too, but I've not done that)
• There's no animation glitch on refresh #14251
• The ghost cell does eventually go away now (though it could still do with one less load cycle)

Steps to reproduce

1. Launch the app and go to POS
2. Observe that the initial loading state is shown
3. When the items load, observe that they're shown correctly
4. Scroll to the bottom of the list; observe that the next page is loaded
5. Pull to refresh; observe that the list refreshes without glitches

Test with a site that has no simple products; observe that the full-screen empty state is shown.

Test with the products endpoint on a network breakpoint, and abort the request; observe that the error state is shown and can be refreshed correctly.

Testing information

This refactor should be limited to the display of the itemlist, but it's worth also checking a full checkout including order editing after the first checkout tap, because the items array is used for building the order.

I've tested this with the above scenarios on an iPad Air running iOS 17.7

Screenshots

https://github.com/user-attachments/assets/4b2001c8-c686-4648-902b-fbffd35492a3

---

• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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• [x] Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• [x] Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr14362-a7194c9
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	a7194c9207bb9e038812f7d806e0dfe9719c66db
	App Center Build	WooCommerce - Prototype Builds #11508

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[joshheald]

@iamgabrielma thanks for the review.

I couldn't replicate the case for the ghost cell eventually disappearing when we reach the end of the infinite scroll, it always remains there, but I see it working on your recording. Not really part of this PR, but from what I see the state keeps refreshing when we've reached the end of the list, so it keeps switching between setting isLoadingAfterInitialLoad to false, and then true, which renders the GhostItemCardView indefinitely. I can tackle this separately, just asking in case you've experienced a different behaviour.

Strange, but yeah I definitely see it disappear eventually. There may be some iOS differences? But mine is on iOS 17.7 so not far away from your versions...