applicationId uz neni povinne

Credentials/IdokladCredentials.cs

@@ -10,6 +10,13 @@ public sealed record IdokladCredentials
 {
     public required string ClientId { get; init; }
     public required string ClientSecret { get; init; }
-    public required string ApplicationId { get; init; }
+
+    /// <summary>
+    /// Optional iDoklad ApplicationId (GUID from the developer portal). Only needed for partner
+    /// applications whose client_credentials grant requires application_id. When empty, the
+    /// service authenticates with client_id + client_secret only.
+    /// </summary>
+    public string? ApplicationId { get; init; }
+
     public required Language Language { get; init; }
 }

Credentials/RequestCredentialsProvider.cs

@@ -28,12 +28,12 @@ public sealed class RequestCredentialsProvider
 
         var clientId = HeaderOrDefault(headers, CredentialConstants.ClientIdHeader, _settings.ClientId);
         var clientSecret = HeaderOrDefault(headers, CredentialConstants.ClientSecretHeader, _settings.ClientSecret);
+        // ApplicationId is optional: iDoklad's standard client_credentials grant does not require it.
         var applicationId = HeaderOrDefault(headers, CredentialConstants.ApplicationIdHeader, _settings.ApplicationId);
 
         var missing = new List<string>();
         if (string.IsNullOrWhiteSpace(clientId)) missing.Add(CredentialConstants.ClientIdHeader);
         if (string.IsNullOrWhiteSpace(clientSecret)) missing.Add(CredentialConstants.ClientSecretHeader);
-        if (string.IsNullOrWhiteSpace(applicationId)) missing.Add(CredentialConstants.ApplicationIdHeader);
         if (missing.Count > 0)
         {
             throw new MissingCredentialsException(missing);
@@ -43,7 +43,7 @@ public sealed class RequestCredentialsProvider
         {
             ClientId = clientId!,
             ClientSecret = clientSecret!,
-            ApplicationId = applicationId!,
+            ApplicationId = string.IsNullOrWhiteSpace(applicationId) ? null : applicationId,
             Language = ResolveLanguage(headers),
         };
     }