Refactor error handling in DocumentService

Centralized `HttpRequestException` creation with a new
`CreateHttpException` method to improve code reuse and
consistency. Updated methods to handle `HttpStatusCode.NotFound`
gracefully, returning `null` where appropriate instead of
throwing exceptions. Introduced `TryGetSenderDocumentBytesByEnvelopeIdAsync`
to encapsulate fallback logic for fetching document bytes.
Removed redundant inline exception handling and standardized
error handling across methods.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/DocumentService.cs

@@ -10,6 +10,15 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
 {
     private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
 
+    private static HttpRequestException CreateHttpException(string messagePrefix, HttpStatusCode statusCode, string? reasonPhrase)
+    {
+        var statusCodeInt = (int)statusCode;
+        return new HttpRequestException(
+            $"{messagePrefix}. Status: {statusCodeInt} ({reasonPhrase ?? "Unknown error"})",
+            null,
+            statusCode);
+    }
+
     /// <summary>
     /// Fetches the PDF bytes for the given envelope key from the API.
     /// Throws HttpRequestException on failure with appropriate status code.
@@ -21,14 +30,7 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
         var response = await http.GetAsync($"/api/Document/{Uri.EscapeDataString(envelopeKey)}", cancel);
 
         if (!response.IsSuccessStatusCode)
-        {
+            throw CreateHttpException("Failed to load document", response.StatusCode, response.ReasonPhrase);
-            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;
@@ -43,32 +45,29 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
         using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
         var detailsResponse = await http.GetAsync($"/api/Document/details?EnvelopeId={envelopeId}", cancel);
 
-        if (!detailsResponse.IsSuccessStatusCode)
+        if (detailsResponse.StatusCode == HttpStatusCode.NotFound)
         {
-            var statusCode = (int)detailsResponse.StatusCode;
+            var fallbackBytes = await TryGetSenderDocumentBytesByEnvelopeIdAsync(http, envelopeId, cancel);
-            var reasonPhrase = detailsResponse.ReasonPhrase ?? "Unknown error";
+            if (fallbackBytes is null)
-            throw new HttpRequestException(
+                return null;
-                $"Failed to load sender document details. Status: {statusCode} ({reasonPhrase})",
+
-                null,
+            return new DocumentDto(fallbackBytes, new DocumentDetailsDto
-                detailsResponse.StatusCode);
+            {
+                EnvelopeId = envelopeId,
+                AddedWhen = DateTime.UtcNow,
+                Elements = []
+            });
         }
 
+        if (!detailsResponse.IsSuccessStatusCode)
+            throw CreateHttpException("Failed to load sender document details", detailsResponse.StatusCode, detailsResponse.ReasonPhrase);
+
         var details = await detailsResponse.Content.ReadFromJsonAsync<DocumentDetailsDto>(_jsonOptions, cancel)
             ?? throw new HttpRequestException("Failed to deserialize sender document details response.");
 
-        var bytesResponse = await http.GetAsync($"/api/Document?EnvelopeId={envelopeId}", cancel);
+        var byteData = await TryGetSenderDocumentBytesByEnvelopeIdAsync(http, envelopeId, cancel);
-
+        if (byteData is null)
-        if (!bytesResponse.IsSuccessStatusCode)
+            byteData = [];
-        {
-            var statusCode = (int)bytesResponse.StatusCode;
-            var reasonPhrase = bytesResponse.ReasonPhrase ?? "Unknown error";
-            throw new HttpRequestException(
-                $"Failed to load sender document bytes. Status: {statusCode} ({reasonPhrase})",
-                null,
-                bytesResponse.StatusCode);
-        }
-
-        var byteData = await bytesResponse.Content.ReadAsByteArrayAsync(cancel);
 
         return new DocumentDto(byteData, details);
     }
@@ -82,16 +81,24 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
         using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
         var response = await http.GetAsync($"/api/Document?EnvelopeId={envelopeId}", cancel);
 
+        if (response.StatusCode == HttpStatusCode.NotFound)
+            return null;
+
         if (!response.IsSuccessStatusCode)
-        {
+            throw CreateHttpException("Failed to load sender document bytes", response.StatusCode, response.ReasonPhrase);
-            var statusCode = (int)response.StatusCode;
-            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
-            throw new HttpRequestException(
-                $"Failed to load sender document bytes. Status: {statusCode} ({reasonPhrase})",
-                null,
-                response.StatusCode);
-        }
 
         return await response.Content.ReadAsByteArrayAsync(cancel);
     }
+
+    private static async Task<byte[]?> TryGetSenderDocumentBytesByEnvelopeIdAsync(HttpClient http, int envelopeId, CancellationToken cancel)
+    {
+        var bytesResponse = await http.GetAsync($"/api/Document?EnvelopeId={envelopeId}", cancel);
+        if (bytesResponse.StatusCode == HttpStatusCode.NotFound)
+            return null;
+
+        if (!bytesResponse.IsSuccessStatusCode)
+            throw CreateHttpException("Failed to load sender document bytes", bytesResponse.StatusCode, bytesResponse.ReasonPhrase);
+
+        return await bytesResponse.Content.ReadAsByteArrayAsync(cancel);
+    }
 }