Add ReceiverService and enhance DocumentService

Added `ReceiverService` to the dependency injection container in `Program.cs` and implemented its functionality for fetching receiver email suggestions via API.

Enhanced `DocumentService` with a new method `GetSenderDocumentDetailsByEnvelopeIdAsync` to fetch sender document details by envelope ID. Introduced JSON handling improvements with `JsonSerializerOptions`.

These changes improve support for receiver-related operations and extend document-related capabilities.

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

@@ -1,10 +1,14 @@
 using System.Net;
 using System.Net.Http;
+using System.Net.Http.Json;
+using System.Text.Json;
+using EnvelopeGenerator.Application.Common.Dto;
 
 namespace EnvelopeGenerator.Server.Client.Services;
 
 public class DocumentService(IHttpClientFactory httpClientFactory)
 {
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
 
     /// <summary>
     /// Fetches the PDF bytes for the given envelope key from the API.
@@ -29,4 +33,26 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
         var bytes = await response.Content.ReadAsByteArrayAsync(cancel);
         return bytes;
     }
+
+    /// <summary>
+    /// Fetches sender document details (including signature elements) by envelope id.
+    /// Uses GET /api/Document?EnvelopeId={envelopeId}.
+    /// </summary>
+    public async Task<DocumentDto?> GetSenderDocumentDetailsByEnvelopeIdAsync(int envelopeId, CancellationToken cancel = default)
+    {
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var response = await http.GetAsync($"/api/Document?EnvelopeId={envelopeId}", cancel);
+
+        if (!response.IsSuccessStatusCode)
+        {
+            var statusCode = (int)response.StatusCode;
+            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+            throw new HttpRequestException(
+                $"Failed to load sender document details. Status: {statusCode} ({reasonPhrase})",
+                null,
+                response.StatusCode);
+        }
+
+        return await response.Content.ReadFromJsonAsync<DocumentDto>(_jsonOptions, cancel);
+    }
 }