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/ReceiverService.cs

@@ -0,0 +1,29 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+public class ReceiverService(IHttpClientFactory httpClientFactory)
+{
+    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web);
+
+    /// <summary>
+    /// Fetches receiver email suggestions from sender-authorized API.
+    /// Calls GET /api/Receiver?EmailAddressSearch={term}&onlyEmailAddress=true.
+    /// </summary>
+    public async Task<IReadOnlyList<string>> SearchReceiverEmailsAsync(string searchTerm, CancellationToken cancel = default)
+    {
+        if (string.IsNullOrWhiteSpace(searchTerm))
+            return [];
+
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var url = $"/api/Receiver?EmailAddressSearch={Uri.EscapeDataString(searchTerm)}&onlyEmailAddress=true";
+        var response = await http.GetAsync(url, cancel);
+
+        if (!response.IsSuccessStatusCode)
+            return [];
+
+        var emails = await response.Content.ReadFromJsonAsync<List<string>>(JsonOptions, cancel);
+        return emails ?? [];
+    }
+}