Refactor error handling in DocumentService and consumers

Refactored `DocumentService.GetDocumentAsync` to throw
`HttpRequestException` on failure instead of returning a tuple.
Updated all consumers to handle exceptions, improving error
handling consistency across the application.

Enhanced error messages for better user feedback and added
logging for improved traceability. Updated `EnvelopeReceiverPage`,
`EnvelopeReceiverPage_DxReportViewer`, and `ReportViewer` to
use the new exception-based API and handle errors gracefully.

This change simplifies the API, improves maintainability, and
makes the application more robust and user-friendly.

EnvelopeGenerator.ReceiverUI/Services/DocumentService.cs

@@ -11,17 +11,25 @@ public class DocumentService(HttpClient http, IOptions<ApiOptions> apiOptions)
 
     /// <summary>
     /// Fetches the PDF bytes for the given envelope key from the API.
-    /// Returns null bytes with the HTTP status code on failure.
+    /// Throws HttpRequestException on failure with appropriate status code.
     /// </summary>
-    public async Task<(byte[]? Bytes, HttpStatusCode StatusCode)> GetDocumentAsync(string envelopeKey, CancellationToken cancel = default)
+    /// <exception cref="HttpRequestException">Thrown when the API request fails.</exception>
+    public async Task<byte[]?> 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 statusCode = (int)response.StatusCode;
+            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+            throw new HttpRequestException(
+                $"Failed to load document. Status: {statusCode} ({reasonPhrase})",
+                null,
+                response.StatusCode);
+        }
 
         var bytes = await response.Content.ReadAsByteArrayAsync(cancel);
-        return (bytes, response.StatusCode);
+        return bytes;
     }
 }