Field naming inconsistency: last_4 in webhooks vs last4 in API responses

### Field naming inconsistency: `last_4` in webhooks vs `last4` in API responses

### Environment
* Checkout SDK version: N/A (API inconsistency)
* Platform and version: PHP 8.x
* Operating System and version: All platforms

### Description
Checkout.com uses inconsistent field names for card's last four digits:
- Webhook events use `last_4` (with underscore)
- REST API responses use `last4` (no underscore)

### Expected behavior
Field name should be consistent across all endpoints - either `last4` or `last_4` everywhere.

### Current behavior
**Webhook event (`card_verified`):**
```php
"source" => [
  "last_4" => "0018"  // With underscore
]
```

**API response (`GET /payments/{id}`):**
```php
"source" => [
  "last4" => "0018"  // No underscore
]
```

### Steps to reproduce
1. Receive a `card_verified` webhook - observe `last_4` field
2. Call `GET /payments/{id}` - observe `last4` field
3. Note the inconsistency

### Possible solution
Standarize on one naming convention (preferably `last_4` with underscore) across all endpoints. Provide deprecation period for the old field name.

**Current workaround required:**
```php
$lastFour = $data['last_4'] ?? $data['last4'] ?? null;
```

### Additional information
This affects all integrations using both webhooks and REST API. All other card fields (expiry_month, fingerprint, etc.) are consistent - only last4/last_4 has this issue.

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Field naming inconsistency: last_4 in webhooks vs last4 in API responses #297