[AC-1800] PayPal IPN Refactor (#3619)

* Add more logging to PayPal IPN webhook

* Add PayPalIPNClient tests

* Add PayPalControllerTests

---------

Co-authored-by: aelinton <95626935+aelinton@users.noreply.github.com>

src/Billing/Services/Implementations/PayPalIPNClient.cs

@@ -0,0 +1,86 @@
+using System.Text;
+using Microsoft.Extensions.Options;
+
+namespace Bit.Billing.Services.Implementations;
+
+public class PayPalIPNClient : IPayPalIPNClient
+{
+    private readonly HttpClient _httpClient;
+    private readonly Uri _ipnEndpoint;
+    private readonly ILogger<PayPalIPNClient> _logger;
+
+    public PayPalIPNClient(
+        IOptions<BillingSettings> billingSettings,
+        HttpClient httpClient,
+        ILogger<PayPalIPNClient> logger)
+    {
+        _httpClient = httpClient;
+        _ipnEndpoint = new Uri(billingSettings.Value.PayPal.Production
+            ? "https://www.paypal.com/cgi-bin/webscr"
+            : "https://www.sandbox.paypal.com/cgi-bin/webscr");
+        _logger = logger;
+    }
+
+    public async Task<bool> VerifyIPN(Guid entityId, string formData)
+    {
+        LogInfo(entityId, $"Verifying IPN against {_ipnEndpoint}");
+
+        if (string.IsNullOrEmpty(formData))
+        {
+            throw new ArgumentNullException(nameof(formData));
+        }
+
+        var requestMessage = new HttpRequestMessage { Method = HttpMethod.Post, RequestUri = _ipnEndpoint };
+
+        var requestContent = string.Concat("cmd=_notify-validate&", formData);
+
+        LogInfo(entityId, $"Request Content: {requestContent}");
+
+        requestMessage.Content = new StringContent(requestContent, Encoding.UTF8, "application/x-www-form-urlencoded");
+
+        var response = await _httpClient.SendAsync(requestMessage);
+
+        var responseContent = await response.Content.ReadAsStringAsync();
+
+        if (response.IsSuccessStatusCode)
+        {
+            return responseContent switch
+            {
+                "VERIFIED" => Verified(),
+                "INVALID" => Invalid(),
+                _ => Unhandled(responseContent)
+            };
+        }
+
+        LogError(entityId, $"Unsuccessful Response | Status Code: {response.StatusCode} | Content: {responseContent}");
+
+        return false;
+
+        bool Verified()
+        {
+            LogInfo(entityId, "Verified");
+            return true;
+        }
+
+        bool Invalid()
+        {
+            LogError(entityId, "Verification Invalid");
+            return false;
+        }
+
+        bool Unhandled(string content)
+        {
+            LogWarning(entityId, $"Unhandled Response Content: {content}");
+            return false;
+        }
+    }
+
+    private void LogInfo(Guid entityId, string message)
+        => _logger.LogInformation("Verify PayPal IPN ({RequestId}) | {Message}", entityId, message);
+
+    private void LogWarning(Guid entityId, string message)
+        => _logger.LogWarning("Verify PayPal IPN ({RequestId}) | {Message}", entityId, message);
+
+    private void LogError(Guid entityId, string message)
+        => _logger.LogError("Verify PayPal IPN ({RequestId}) | {Message}", entityId, message);
+}