Shipping Labels: Fix issue mapping shipments after updating for REST API response

[itsmeichigo]

Closes WOOMOB-1261

Description

This PR fixes an issue with parsing data in the response of update shipment endpoint when using REST API.

The cause of the issue is that the response contains data field, so the mapper mistakes it with the data envelope and expects an object instead of string.

The solution is to use do-catch instead when mapping the response.

Testing steps

• Log in to a test store with Woo Shipping set up using site credentials.
• Navigate to the Orders tab and select a completed order with more than 1 physical products.
• Select Split shipments and move items around.
• Tap Done to save the update.
• Confirm that saving succeeds and the Split shipments screen is dismissed.

Testing information

Confirmed the fix with simulator iPhone 16 iOS 18.2 and updated the incorrect unit test.

Screenshots

Before:

Simulator.Screen.Recording.-.iPhone.16.-.2025-09-04.at.12.15.42.mp4

After:

Simulator.Screen.Recording.-.iPhone.16.-.2025-09-04.at.11.58.09.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.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 23.1 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[wpmobilebot]

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

	App Name	WooCommerce iOS Prototype
	Build Number	pr16078-375c97a
	Version	23.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	375c97a
	Installation URL	32res3edcv3s0

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

[RafaelKayumov]

LGTM. Confirming the fix resolves shipment updates when logged in with shipment credentials.