[Woo POS] Extract items code from aggregate model

[joshheald]

Closes: #14496

Description

In this PR we extract the Item list code to a service.

It's quite large but almost all of that is a lift-and-shift of the tests.

Testing information

The main things to check are item list functions – initial loading, pull to refresh, pagination, and error states. However, none of the code has really changed to enable us to extract it, it's close to a pure lift-and-shift.

I tested on an iPad Air running iOS 17.7

Screenshots

extract-items-code-to-service.mp4

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• 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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Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• 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	pr14498-245fa7e
	Version	21.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	245fa7e
	App Center Build	WooCommerce - Prototype Builds #11736

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