Refactor subscription migration system to fix infinite loop, add state tracking and HPOS support

[amans2k]

Problem

The current subscription migration system (class-angelleye-paypal-ppcp-migration.php) has critical issues:

1. Infinite loop bug — Subscriptions with missing payment tokens (e.g., empty _payment_tokens_id, _ppec_billing_agreement_id) are reprocessed indefinitely since there's no mechanism to mark them as permanently failed.
2. No state tracking — No lifecycle states for migration progress; impossible to distinguish pending vs completed vs failed subscriptions.
3. Order note spam — Each failed retry adds duplicate notes, polluting subscription history with thousands of identical entries.
4. No HPOS compatibility — Migration queries don't support WooCommerce High-Performance Order Storage.
5. Monolithic architecture — All logic in a single 648-line class with no separation of concerns, no dependency injection, and no tests.

Solution

Refactor into a modular service-based architecture:

• Migration_Controller — Facade/orchestrator with public API (start, stats, retry, reset)
• Subscription_Migration_Service — Core migration logic with proper error handling
• Action_Scheduler_Batch_Processor — Queue management with batch limits and delays
• Payment_Token_Validator — Validates 4 token meta keys with format checking
• Payment_Method_Updater — Handles payment method switching
• HPOS_Migration_State_Storage — HPOS-compatible state persistence
• Migration_Status enum — Categorized statuses: COMPLETED, FAILED_NO_TOKEN, FAILED_API_ERROR, FAILED_DATA_ERROR, SKIPPED_EXCLUDED, SKIPPED_MANUAL
• DTOs — Migration_Result, Batch_Result, Migration_Stats
• Migration_Admin_Page — Dashboard with real-time progress, failure counts by type, retry/reset controls
• Test suite — Unit and integration tests

Key behavioral changes

• Subscriptions get lifecycle states: NOT_STARTED → IN_PROGRESS → COMPLETED/FAILED_*
• Failed subscriptions are categorized and skipped on next run (no infinite loop)
• Order notes only added on first failure attempt
• HPOS auto-detection for compatible storage queries