Ensure push notifications re-register after store switch

[RafaelKayumov]

Description

Runtime store switches were leaving the device unsubscribed because DefaultStoresManager.removeDefaultStore() always called unregisterForRemoteNotifications() while updateDefaultStore(storeID:) never re-registered, so users missed pushes until the next cold start. We now invoke a new helper, registerForPushNotificationsIfNeeded(), from updateDefaultStore(storeID:) to repeat the registration flow immediately (including simulator-friendly test detection via ProcessInfo.isRunningUnitTests). The change is covered by an updated MockPushNotificationsManager plus a new assertion in test_updateDefaultStore_registersForRemoteNotifications.

Test Steps

• Sign in with a WordPress.com account that has at least two Jetpack-connected stores and accept the initial push notification permission prompt.
• Trigger a push-worthy event (e.g., place a test order) for Store A and confirm the device receives the notification while that store is selected.
• In the app, switch to Store B via the store picker.
• Trigger the same kind of event for Store B and confirm the device now receives that notification without needing to relaunch the app or re-enable notifications.
• While exercising the flow, note that no extra push-permission prompt appears and that badge counts continue to update normally.

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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 Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16397-67df2ad
	Version	23.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	67df2ad
	Installation URL	67ep277ol8p4o

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