[native_assets_cli] Nest config extensions

[dcharkes]

Closes: #2091

Nests the extension configurations under an extension key in config:

{
  "config": {
    "code": { // old location
      "android": {
        "target_ndk_api": 21
      },
      "link_mode_preference": "dynamic"
    },
    "extensions": {
      "code_assets": { // new location, scoped by package name
        "android": {
          "target_ndk_api": 21
        },
        "link_mode_preference": "dynamic"
      }
    }
  },
}

PR changes

• pkgs/hook and pkgs/code_assets

  • Introduces new schemas in the non-generated .schema.json files.
  • Adds deprecation and version skew notice in schemas. (And update the generated schema to pick these comments up.)
  • Adds test data for the new location.
  • Added JSON schema tests for the new location

• package:native_assets_cli

  • Generated the syntax classes.
  • Deal with version skew in the mapping between syntax classes and semantic API.
  • Fix tests that deal with syntax errors.

Version skew between hooks and SDKs

• Backwards compatibility older hooks: Still emit config.code.
• Backwards compatibility older SDKs: Also read config.extensions.code_config.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

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: 85.954% (-0.07%) from 86.021%
when pulling 2c05819 on nest-config-extensions
into e2bd08e on main.

[dcharkes]

Friendly ping @mosuem @HosseinYousefi. This one in the big stack was left unreviewed.