Add DocumentService for fetching PDF documents

Introduced a new `DocumentService` class to handle fetching
PDF document bytes from an API endpoint. The service uses
`HttpClient` for HTTP communication and `IOptions<ApiOptions>`
for accessing API configuration. Added the `GetDocumentAsync`
method to perform the HTTP GET request, handle responses, and
return the document bytes along with the HTTP status code.
Included necessary `using` directives and encapsulated the
service in the `EnvelopeGenerator.ReceiverUI.Services` namespace.

EnvelopeGenerator.ReceiverUI/Services/DocumentService.cs

@@ -0,0 +1,27 @@
+using System.Net;
+using System.Net.Http;
+using Microsoft.Extensions.Options;
+using EnvelopeGenerator.ReceiverUI.Options;
+
+namespace EnvelopeGenerator.ReceiverUI.Services;
+
+public class DocumentService(HttpClient http, IOptions<ApiOptions> apiOptions)
+{
+    private readonly ApiOptions _api = apiOptions.Value;
+
+    /// <summary>
+    /// Fetches the PDF bytes for the given envelope key from the API.
+    /// Returns null bytes with the HTTP status code on failure.
+    /// </summary>
+    public async Task<(byte[]? Bytes, HttpStatusCode StatusCode)> GetDocumentAsync(string envelopeKey, CancellationToken cancel = default)
+    {
+        var response = await http.GetAsync($"{_api.BaseUrl}/api/Document/{Uri.EscapeDataString(envelopeKey)}", cancel);
+
+        if (!response.IsSuccessStatusCode)
+            return (null, response.StatusCode);
+
+        var bytes = await response.Content.ReadAsByteArrayAsync(cancel);
+        return (bytes, response.StatusCode);
+    }
+}
+