Refactor exchange rate providers into driver-based architecture

Replace duplicated switch/case blocks across 4 methods with a clean
abstract driver pattern:

- ExchangeRateDriver (abstract): defines getExchangeRate(),
  getSupportedCurrencies(), validateConnection()
- CurrencyFreakDriver, CurrencyLayerDriver, OpenExchangeRateDriver,
  CurrencyConverterDriver: concrete implementations
- ExchangeRateDriverFactory: resolves driver name to class, with
  register() method for module extensibility

Delete ExchangeRateProvidersTrait — all logic now lives in driver
classes and ExchangeRateProviderService. Adding a new exchange rate
provider only requires implementing ExchangeRateDriver and calling
ExchangeRateDriverFactory::register() in a module service provider.

app/Services/ExchangeRate/ExchangeRateException.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace App\Services\ExchangeRate;
+
+class ExchangeRateException extends \RuntimeException
+{
+    public function __construct(string $message, public readonly string $errorKey = 'exchange_rate_error')
+    {
+        parent::__construct($message);
+    }
+}