Move conditional includes up one level to avoid layer violations

Author: gspencergoog

packages/a2a_dart/lib/a2a_dart.dart

@@ -21,6 +21,7 @@ export 'src/core/security_scheme.dart';
 export 'src/core/task.dart';
 // Server exports (conditionally exported).
 export 'src/server/a2a_server.dart';
+export 'src/server/a2a_server_exception.dart';
 export 'src/server/create_task_handler.dart';
 export 'src/server/handler_result.dart';
 export 'src/server/request_handler.dart';

packages/a2a_dart/lib/src/server/a2a_server.dart

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-export 'web/a2a_server.dart' if (dart.library.io) 'io/a2a_server.dart';
+export 'web_a2a_server.dart' if (dart.library.io) 'io_a2a_server.dart';