Always add currency to cart (6272)

[stracker-phil]

This PR extends #4306

Description

The totals calculation requires a valid currency code to work, as a money value without a currency code is invalid.

Current Behavior

• The Cart API waits until PayPal provides a currency.
• This does not happen in early requests, and as a result we do not include price or currency details in the response
• Postman collection: Expects to see minimal cart totals immediately after an item was added

New Behavior

• When there is no currency code defined in the incoming PayPal cart, the Cart API falls back to use Woo's currency
• This unlocks totals calculation as soon as an item is in the cart
• Postman collection: Confirms that our API is working as expected
• Consequence: The AI agent can infer the currency and product price from the response, instead of relying on the product catalog

[github-actions]

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What's included:

• ✅ WordPress (latest)
• ✅ WooCommerce (latest)
• ✅ PayPal Payments plugin v4.0.2-pr4311-25164922827-g897e398 (built from this PR)

Login credentials:

• Username: admin
• Password: password

Plugin Details:

• Version: 4.0.2-pr4311-25164922827-g897e398
• Commit: e736ad8
• Artifact: woocommerce-paypal-payments-4.0.2-pr4311-25164922827-g897e398

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[danieldudzic]

Thanks for working on this. Just a few minor issues.