Add Subscription Product Helpers/Abstractions

[agibson-godaddy]

Summary

Adds a new SubscriptionProductHelper to the framework that centralizes subscription-product detection ("does this product have a subscription?") and detail lookups (period, length, trial, sign-up fee, expiration date), so plugins get correct answers whether a store is running legacy WooCommerce Subscriptions product types or WC Subscriptions 9.0+'s bundled "subscription scheme" mechanism.

Story: MWC-20234

Release: #853

Details

WC Subscriptions 9.0 merged "All Products for Subscriptions" (APFS) into core, adding a second way to sell a subscription: any Simple/Variable/etc. product can carry one or more subscription "schemes" without changing its product type. The existing WC_Subscriptions_Product::is_subscription() (and its sibling getters) only ever check the legacy product-type path, so a scheme-based product silently reports as "not a subscription" wherever that method is used on its own — this is the root cause behind MWC-20228 in WooCommerce Memberships.

Rather than fixing this as a one-off in Memberships, this PR moves the generic detection/read logic into the framework so any plugin built on it benefits:

• Detection and lookups work correctly across both mechanisms out of the box.
• Compatibility is gated by class/method presence rather than a Subscriptions version number, so it stays correct whether a store runs WC Subscriptions 9+ (schemes bundled) or an older WC Subscriptions with the standalone APFS plugin installed alongside it.
• Consuming plugins (starting with Memberships via MWC-20228) can drop their own duplicate detection code and call the shared helper instead.

Includes unit test coverage for both mechanisms and the scheme fallback behavior (active → default → base scheme).

QA

TODO - test with memberships