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/ExchangeRateDriver.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Services\ExchangeRate;
+
+abstract class ExchangeRateDriver
+{
+    public function __construct(
+        protected string $apiKey,
+        protected array $config = [],
+    ) {}
+
+    /**
+     * Fetch exchange rate between two currencies.
+     *
+     * @return array Exchange rate values
+     */
+    abstract public function getExchangeRate(string $baseCurrency, string $targetCurrency): array;
+
+    /**
+     * Get list of currencies supported by this provider.
+     *
+     * @return array Currency codes
+     */
+    abstract public function getSupportedCurrencies(): array;
+
+    /**
+     * Validate that the API key and connection work.
+     *
+     * @return array Exchange rate values (proof of working connection)
+     *
+     * @throws ExchangeRateException
+     */
+    abstract public function validateConnection(): array;
+}