[native_assets_cli] Use JSON enums where applicable

[dcharkes]

Enums in JSON schemas are more concise and more apt than a list of consts.

Thanks @jonasfj for the tip!

(Also updates the schema analyzer to recognize them.)

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart