Add validation to prevent refunds with invalid amounts

Adds check to ensure refund amount is greater than 0 before calling wc_create_refund(). This prevents "Invalid refund amount" errors that occur when:
- Orders have 100% discount coupons (total = $0)
- Line items have $0 totals
- Calculation results in 0 or negative amount

Logs order ID, calculated amount, and order total when skipping refund.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: layoutd

includes/Generator/Order.php

@@ -521,6 +521,17 @@ protected static function create_refund( $order, $force_partial = false ) {
 			}
 		}
 
+		// Validate refund amount is greater than 0
+		if ( $refund_amount <= 0 ) {
+			error_log( sprintf(
+				'Refund skipped for order %d: Invalid refund amount (%s). Order total: %s',
+				$order->get_id(),
+				$refund_amount,
+				$order->get_total()
+			) );
+			return false;
+		}
+
 		// Create refund reason
 		if ( $is_full_refund ) {
 			$reason = 'Full refund';