Add multi-gateway support

[dparker1005]

Summary

Adds the ability to enable multiple payment gateways simultaneously so members can choose their preferred payment method at checkout. This is a v4.0 breaking change — custom checkout templates will need to be updated.

Core Infrastructure

• New pmpro_get_enabled_gateways() function with automatic migration from the legacy single pmpro_gateway option
• New static methods on PMProGateway base class: get_checkout_label(), enqueue_checkout_scripts(), show_checkout_fields(), requires_billing_address(), get_required_billing_fields(), show_submit_button() — with legacy hook detection for gateways that haven't been updated
• pmpro_getGateway() marked as deprecated

Admin UI

• Payment Settings page redesigned with sortable Active Gateways table, collapsible Available Gateways panel, and activate/deactivate buttons
• Gateway order determines checkout display order (first = default)

Checkout & Billing

• Gateway selector radio buttons when multiple gateways enabled
• Per-gateway payment field containers and submit buttons with JS toggling
• Billing address visibility respects each gateway's requires_billing_address()
• Free-level fallback submit button for discount-code-to-free scenarios
• Billing page updated to use gateway static methods

Gateway Updates (Stripe, Check, PayPal Express)

• Each overrides the new static methods for gateway-specific behavior
• Stripe: SCA intent localization preserved, Stripe Checkout (offsite) handled
• PayPal Express: offsite redirect button with free-level fallback

Backward Compatibility

• Base class detects legacy filter callbacks via hooked filters AND method_exists() (covers secondary gateways whose init() didn't fire)
• pmpro_gateway option kept in sync for legacy code
• Pay by Check add-on UI deferred via pmpro_show_gateway_selector filter
• Add PayPal Express add-on added to deprecated add-ons list (auto-deactivated)

Open TODOs

• Live price updates at checkout — Could simplify the show/hide logic for payment fields when a discount code makes a level free/paid, replacing the current applydiscountcode.php JS approach
• Per-level gateway configuration — The Pay by Check add-on currently allows enabling/disabling gateways per level. How should core support this? Per-level gateway overrides in the level editor?
• Address for Free Levels Add On — How should this function with multi-gateway? Per-gateway toggle to force billing address? A sitewide checkbox? Currently requires_billing_address() is per-gateway but there's no admin-facing override
• Duplicate DOM IDs — When two onsite gateways are enabled, each gateway's show_checkout_fields() renders elements with the same IDs. Gateway JS that binds by ID hits the wrong element. Needs a namespacing strategy
• Deprecated gateways as secondary — Deprecated gateways (Braintree, etc.) render fields but not scripts when used as secondary gateways. Their init() only hooks checkout actions when they're the active gateway. They would need enqueue_checkout_scripts() overrides to work as secondary gateways
• Discount code AJAX — Doesn't send the selected gateway parameter, so the server response can't be gateway-aware
• Replace all @since TBD tags with actual version number before release

Test Plan

• Fresh install: single gateway, no radio buttons, behavior identical to current
• Enable Stripe + Check: radio buttons appear, fields toggle, both checkout flows complete
• Enable Stripe + PayPal Express: on-site fields for Stripe, redirect button for PayPal
• Stripe Checkout (offsite) + Check: no card fields for Stripe, check shows instructions
• Stripe SCA/3D Secure: verify authentication flow completes after redirect
• Discount code makes level free: gateway selector hides, free submit button shows
• Discount code removed (level paid again): gateway selector restores, correct fields show
• Update billing: uses original subscription gateway, correct fields render
• Admin: activate/deactivate gateways, drag to reorder, verify save persists order
• Admin: deactivate all gateways, verify testing mode notice
• Pay by Check add-on active: core selector deferred, PBC radio buttons used instead
• Add PayPal Express add-on: auto-deactivated with admin notice

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