Fix inconsistent subscription status after cancellation with centralized cancellation logic

[lmcrean]

Problem Statement

After cancelling a subscription, the status is inconsistent across different areas (#78):

• Backend Orders: shows "pending subscription" (Wrong)
• My Account > Subscriptions: shows "pending subscription" (Wrong)
• My Account > Orders: shows "cancelled" (Correct)

Root Cause

Subscription cancellation events weren't properly synchronized between orders, subscriptions, and renewal orders. The gateway was missing status propagation logic.

Solution

Add centralized cancellation handling and status synchronization:

1. Unified cancellation handler - Single method to handle all cancellation scenarios (payment cancelled, billing request cancelled, subscription cancelled)

2. Status synchronization hooks - Ensure parent orders reflect subscription status changes and vice versa

3. Metadata cleanup - Clear pending payment flags when cancellation occurs

Changes

• class-wc-gocardless-gateway.php: Add handle_subscription_cancellation() method
• class-wc-gocardless-gateway-addons.php: Add sync_parent_order_status() hook
• Update webhook handlers to use centralized cancellation logic

Testing

1. Create subscription order
2. Cancel subscription from frontend/backend
3. Verify all areas show "cancelled" status consistently

Unit Test Instructions

# Install dependencies
composer install

# Run cancellation tests
vendor/bin/phpunit --testdox --filter="Test_Cancellation_Simplified|Test_Parent_Order_Sync"

Expected: 13/13 tests pass validating subscription cancellation logic

[lmcrean]

Confirmed in unit testing, does anyone have access to trying on local machine with the paid subscription plugin? @diegocurbelo @peterwilsoncc @joemcgill @dkotter