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/CurrencyLayerDriver.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace App\Services\ExchangeRate;
+
+use Illuminate\Support\Facades\Http;
+
+class CurrencyLayerDriver extends ExchangeRateDriver
+{
+    private string $baseUrl = 'http://api.currencylayer.com';
+
+    public function getExchangeRate(string $baseCurrency, string $targetCurrency): array
+    {
+        $url = "{$this->baseUrl}/live?access_key={$this->apiKey}&source={$baseCurrency}&currencies={$targetCurrency}";
+        $response = Http::get($url)->json();
+
+        if (array_key_exists('success', $response) && $response['success'] == false) {
+            throw new ExchangeRateException($response['error']['info'], 'provider_error');
+        }
+
+        return array_values($response['quotes']);
+    }
+
+    public function getSupportedCurrencies(): array
+    {
+        $url = "{$this->baseUrl}/list?access_key={$this->apiKey}";
+        $response = Http::get($url)->json();
+
+        if ($response == null) {
+            throw new ExchangeRateException('Server not responding', 'server_error');
+        }
+
+        if (array_key_exists('currencies', $response)) {
+            return array_keys($response['currencies']);
+        }
+
+        throw new ExchangeRateException('Please Enter Valid Provider Key.', 'invalid_key');
+    }
+
+    public function validateConnection(): array
+    {
+        $url = "{$this->baseUrl}/live?access_key={$this->apiKey}&source=INR&currencies=USD";
+        $response = Http::get($url)->json();
+
+        if (array_key_exists('success', $response) && $response['success'] == false) {
+            throw new ExchangeRateException($response['error']['info'], 'provider_error');
+        }
+
+        return array_values($response['quotes']);
+    }
+}