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/OpenExchangeRateDriver.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace App\Services\ExchangeRate;
+
+use Illuminate\Support\Facades\Http;
+
+class OpenExchangeRateDriver extends ExchangeRateDriver
+{
+    private string $baseUrl = 'https://openexchangerates.org/api';
+
+    public function getExchangeRate(string $baseCurrency, string $targetCurrency): array
+    {
+        $url = "{$this->baseUrl}/latest.json?app_id={$this->apiKey}&base={$baseCurrency}&symbols={$targetCurrency}";
+        $response = Http::get($url)->json();
+
+        if (array_key_exists('error', $response)) {
+            throw new ExchangeRateException($response['description'], $response['message']);
+        }
+
+        return array_values($response['rates']);
+    }
+
+    public function getSupportedCurrencies(): array
+    {
+        $url = "{$this->baseUrl}/currencies.json";
+        $response = Http::get($url)->json();
+
+        if ($response == null) {
+            throw new ExchangeRateException('Server not responding', 'server_error');
+        }
+
+        $checkKey = $this->validateConnection();
+
+        return array_keys($response);
+    }
+
+    public function validateConnection(): array
+    {
+        $url = "{$this->baseUrl}/latest.json?app_id={$this->apiKey}&base=INR&symbols=USD";
+        $response = Http::get($url)->json();
+
+        if (array_key_exists('error', $response)) {
+            if ($response['status'] == 401) {
+                throw new ExchangeRateException('Please Enter Valid Provider Key.', 'invalid_key');
+            }
+
+            throw new ExchangeRateException($response['description'], $response['message']);
+        }
+
+        return array_values($response['rates']);
+    }
+}