Implement comprehensive Stripe invoice payment webhook processing with multi-service integration

Overview Implements the processInvoicePaymentSucceeded method to handle Stripe invoice payment webhook events with comprehensive data resolution and multi-service coordination.

Why This MR is Large This feature required implementing a complete end-to-end payment processing pipeline that touches multiple services and handles various edge cases:

• Multi-service integration: PaymentServiceClient, SubscriptionServiceClient, and SubscriptionCache
• Robust data resolution: Multiple fallback strategies when webhook data is incomplete
• Direct Stripe API integration: HTTP client utilities for missing data retrieval

Implementation Approach The code is organized into logical, focused helper methods for maintainability:

• Payment data extraction (4 methods)
• Subscription resolution (3 methods)
• Transaction/invoice operations (4 methods)
• Direct Stripe API integration (8 methods)
• Main orchestration method

Services Modified

• PaymentServiceClient: Added createTransaction() and updateInvoice() methods
• SubscriptionServiceClient: Added findSubscriptionIdByGatewayId() and updateSubscription() methods
• SubscriptionCache: Added getSubscriptionForCustomer() for fast lookups
• Main Service: Complete webhook handler with 15+ helper methods

Key Features Implemented

• Payment processing: Processes successful invoice payments from Stripe webhooks
• Data extraction & resolution: Extracts payment details with multiple fallback strategies
• Transaction management: Creates comprehensive transaction records with payment method details
• Invoice/subscription updates: Updates internal records for both subscription and standalone scenarios
• Zero-amount filtering: Skips trial invoices appropriately
• Customer-subscription mapping: Leverages cache for performance

Closes IA-521