Add ReceiverId claim handling to ReceiverClaimExtensions

Updated ReceiverClaimExtensions to include a new array,
`ReceiverIdClaimTypes`, for receiver ID claim types. Added
`GetReceiverIdOfReceiver` method to retrieve and validate
receiver ID claims. Modified the exception message in
`GetEnvelopeIdOfReceiver` to use a string literal for
clarity.

EnvelopeGenerator.API/Extensions/ReceiverClaimExtensions.cs

@@ -12,6 +12,7 @@ namespace EnvelopeGenerator.API.Extensions;
 public static class ReceiverClaimExtensions
 {
     private static readonly string[] EnvelopeIdClaimTypes = [EnvelopeClaimTypes.Id, "envelope_id", "EnvelopeId"];
+    private static readonly string[] ReceiverIdClaimTypes = ["receiver_id", "ReceiverId"];
     private static readonly string[] EnvelopeUuidClaimTypes = [ClaimTypes.NameIdentifier, "envelope_uuid", "EnvelopeUuid"];
     private static readonly string[] ReceiverSignatureClaimTypes = [ClaimTypes.Hash, "receiver_sig", "ReceiverSignature"];
 
@@ -81,12 +82,29 @@ public static class ReceiverClaimExtensions
         var envIdStr = user.GetRequiredClaimOfReceiver(EnvelopeIdClaimTypes);
         if (!int.TryParse(envIdStr, out var envId))
         {
-            throw new InvalidOperationException($"Claim '{EnvelopeClaimTypes.Id}' is not a valid integer.");
+            throw new InvalidOperationException($"Claim '{"envelope_id"}' is not a valid integer.");
         }
 
         return envId;
     }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="user"></param>
+    /// <returns></returns>
+    /// <exception cref="InvalidOperationException"></exception>
+    public static int GetReceiverIdOfReceiver(this ClaimsPrincipal user)
+    {
+        var rcvIdStr = user.GetRequiredClaimOfReceiver(ReceiverIdClaimTypes);
+        if (!int.TryParse(rcvIdStr, out var rcvId))
+        {
+            throw new InvalidOperationException($"Claim '{"receiver_id"}' is not a valid integer.");
+        }
+
+        return rcvId;
+    }
+
     /// <summary>
     /// Signs in an envelope receiver using cookie authentication and attaches envelope claims.
     /// </summary>