Product Form: Show clear error message for duplicated GTIN/UPC/EAN/ISBN

[itsmeichigo]

Closes WOOMOB-1349

Description

This PR catches the error of duplicated GTIN/UPC/EAN/ISBN when saving products and show a user-friend error message instead of generic message.

Testing steps

1. Log in to a store with existing products.
2. Edit a product
3. Scan or manually enter a GTIN/UPC/EAN/ISBN that already exists on another product/variation
4. Attempt to save the changes
5. Confirm that a clear error message is displayed.

Testing information

Tested with simulator iPhone 17 iOS 26.

Screenshots

Before	After

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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 23.3. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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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	pr16146-1bf7cb3
	Version	23.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	1bf7cb3
	Installation URL	38c7vpehjojko

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