[native_assets_cli] Cleanup JSON: old user defines

[dcharkes]

Bug:

• [native_assets_cli] Remove fallback JSON encodings #2199

Cleanup of:

• [native_assets_cli] Add workspace path to input #2187

The old JSON encoding was only replaced one week ago, but it was only in the SDK for a week. So remove the fallback encoding now anyways. If anyone adopted it last week, they know where to update it.

This PR also adds tests for the JSON schema, that were missing:

• user_defines is optional, there might not be any
  • pub_workspace is optional, there might be SDKs without a pubspec.
    • base_path is required. If there are any user-defines, they might have relative paths in them.
    • defines is required. If there are no defines, then the whole pub_workspace should be omitted. (Or simply add an empty map.)

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

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

[coveralls]

coverage: 77.244% (+0.09%) from 77.155%
when pulling 583dd95 on json-cleanup-user-defines
into c159731 on main.

[goderbauer]

LGTM