[Product Creation with AI] Update prompt to generate 3 short descriptions

[selanthiraiyan]

Closes: #13265

Description

Update AI product generation code to generate three options for product short description fields.

Changes

• Make AIProduct's shortDescription property as an array of strings.
• Update AI prompt to ask for three short descriptions.
• Update mock json files and unit tests.

Testing information

1. Test that the M1 of product creation using AI flow works as expected.
2. Intercept the product generation network request and make sure the JSON response contains 3 product short descriptions.
3. Disable the productCreationAIv2M1 feature flag, and make sure the previous AI flow works as before.

Screenshots

NA

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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.

[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	pr13268-b0996ec
	Version	19.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	b0996ec
	App Center Build	WooCommerce - Prototype Builds #9931

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

[itsmeichigo]

LGTM