[Woo POS] Extract cart to aggregate model

[joshheald]

Closes: #14387

Description

This PR extracts the responsibility for the cart contents to the aggregate model.

Some state/actions remain split between the cart and the dashboard; these interact with OrderStage, which will be extracted later.

Testing information

This requires general testing that the cart can be manipulated as expected; adding, removing, clearing, creating an order.

I've tested it across various scenarios on an iPad Air running 17.7

Screenshots

https://github.com/user-attachments/assets/4472ad1f-e77a-45b1-8dc5-783023919840

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

Reviewer (or Author, in the case of optional code reviews):

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

• [x] 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.
• [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	pr14400-08dd345
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	08dd345093b1eb9e2da51d1e76d64c02d1649532
	App Center Build	WooCommerce - Prototype Builds #11560

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