Insonsistent PayPal button behavior for subscriptions on cart (6639)

[Dinamiko]

The PayPal button behaved inconsistently for subscription carts. On the block cart it showed for one subscription product but appeared then disappeared when a second (free-trial) one was added; on the classic cart the opposite happened (hidden for one, shown for two). Separately, emptying the block cart left the mini-cart still showing products.

PR Changes

• Centralized the button-display rule into a single shared decision SubscriptionHelper::paypal_subscription_button_allowed() (PHP) and paypalSubscriptionButtonAllowed() (JS), exposed as one subscription_button_allowed server flag. It covers both modes: PayPal Subscriptions requires a single item with a PayPal plan; vaulting requires vaulting enabled; non-subscription carts are always allowed.
• All three cart paths (classic, block, mini-cart) now consume this one rule, so they behave identically.
• The button is decided before rendering and the wrapper is withheld server-side for unsupported carts (no flicker), and truly hidden rather than greyed.
• The mini-cart now subscribes to wc/store/cart, clearing the button when the block cart empties and re-applying the same subscription rule.

[github-actions]

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What's included:

• ✅ WordPress (latest)
• ✅ WooCommerce (latest)
• ✅ PayPal Payments plugin v4.1.1-pr4542-29812514245-g6caf077 (built from this PR)

Login credentials:

• Username: admin
• Password: password

Plugin Details:

• Version: 4.1.1-pr4542-29812514245-g6caf077
• Commit: 5207862
• Artifact: woocommerce-paypal-payments-4.1.1-pr4542-29812514245-g6caf077

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[Narek13]

Thanks for working on this. Left a small suggestion

[Narek13]

This fires on every store change, not just wc/store/cart.
subscribe() accepts an optional storeNameOrDescriptor second argument to scope the listener to one store like wp.data.subscribe(callback, 'wc/store/cart')