[Shipping labels] Init shipping service separately from loading the label rates

[rachelmcr]

Part of: #14522

Description

We want to display the selected package from the "Add Package" section on the main screen in the Woo Shipping label creation flow. Once the package is selected, we will be able to change the package or update its weight. Any change to the selected package will update the available rates in the shipping service.

This PR is the first step for doing that: It separates the shipping service VM init method from the action to load label rates. That way we can init the VM once and load the label rates each time there are updates to the selected package.

How

• Changes to the shipping service VM (WooShippingServiceViewModel):
  • This removes the selected package from the view model's init method, since it's only used to load the label rates from remote. Any time the selected package changes, we can call the loadLabelRates(for:) method.
  • We add an empty case to the LabelRatesState enum and set that as the default state. We can use this if we ever need to show an empty state for the shipping services.
• Changes to the main screen VM (WooShippingCreateLabelsViewModel):
  • We no longer set the shipping service VM every time selectedPackage is set. Instead, we set it once when the view model is initialized.
  • We have a new method handleNewSelectedPackage(_:) to handle the selected package. It sets the selectedPackage property and also calls loadLabelRates(for:) to get the rates for the package.

Note: We don't yet use the handleNewSelectedPackage(_:) method. The next PR will integrate this into the "Add Package" flow so we can set the selected package.

Testing information

This is a preliminary step in preparation for setting the selected package in the "Add Package" flow. For now, confirm that tests pass (the shipping service works as expected when label rates are loaded).

This can also be tested by manually triggering the handleNewSelectedPackage(_:) method with mock package data.

---

• 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:

• 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.

[dangermattic]

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📲 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	pr14557-35f404d
	Version	21.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	35f404d
	App Center Build	WooCommerce - Prototype Builds #11842

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