[Shipping Labels] Use failsafeDecodeIfPresent for decoding package weight

[bozidarsevo]

Closes: #14751

Description

• Use failsafeDecodeIfPresent for decoding package weight, to reduce code [Shipping Labels] Custom package weight sometimes as box_weight sometimes as boxWeight #14726 (review)

Testing information

1. Install and set up the Woo Shipping extension on your store.
2. Build and run the app with the revampedShippingLabelCreation feature flag enabled.
3. Create an order with the processing status and at least one physical product.
4. In your WooCommerce product settings on the web, change the weight and dimension units and save your changes.
5. On the Orders tab in the app, open the order you created.
6. In the order details, select "Create Shipping Label."
7. Tap "Select a Package."
8. Switch to Carrier and Saved tabs and check that the packages are loaded and showing correct data
9. Switch to Custom tab
10. Fill in all the values for the package, including weight and name and tap "Add Package"
11. Check that on the Create Shipping Labels screen the newly created package is shown, with proper name, dimensions and weight

Screenshots

Simulator.Screen.Recording.-.iPhone.16.Pro.-.2024-12-23.at.11.13.17.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.

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.

[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	pr14753-4c3f659
	Version	21.3
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	4c3f659
	App Center Build	WooCommerce - Prototype Builds #12277

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[bozidarsevo]

Changed it so response example has both box_weight and boxWeight