[native_assets_cli] Cleanup JSON: old asset encoding

[dcharkes]

Bug:

• [native_assets_cli] Remove fallback JSON encodings #2199

Cleanup from:

• [native_assets_cli] JSON nest encoding of assets #2090
• [native_assets_cli] Provide guidance on name spacing extensions #2132

Once this rolls into Dart and Flutter, older hooks will be broken. The oldest version of the hooks that still works is 0.15.0 of native_assets_cli as tested in pkgs/native_assets_builder/test_data/native_add_version_skew/pubspec.yaml.

Once this is used in hooks, those hooks on older SDKs will be broken. Resolving newer versions of this package on older SDKs is prevented by the lower bound SDK constraint.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
native_assets_builder	Breaking	0.16.0	0.17.0-wip	0.17.0	✔️
native_assets_cli	Breaking	0.16.0	0.17.0-wip	0.17.0	✔️

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// 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
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 77.66% (+1.9%) from 75.809%
when pulling 7b6cb42 on json-cleanup-asset-types
into 4e1ef75 on main.