[native_assets_cli] Generate syntax from schemas

[dcharkes]

Bug:

• [native_assets_cli] Use a schema and schema checker for hook config and output #1826

This PR generates a Dart syntax from the JSON schemas for hooks and the extensions.

Moreover, it uses the generated syntax in in package:native_assets_cli to replace the manually written JSON serialization and deserialization.

This PR is meant to be behavior preserving for the current implementation. Any issues with the existing implementation are carried forward, and will be addressed later.

(It is a non-goal of this PR to change the current state.)

Design with Syntax and Semantic API

Before this PR, the semantic API immediately had a toJson and fromJson that took care of any migrations in progress from old to new syntax.

After this PR, the semantic API produces and consumes syntax object (and takes care of migrations in progress), and the syntax does toJson and fromJson. This means we have two layers now instead of one.

The mapping between semantic API and syntax is as follows:

• For classes in the semantic API that occur both in the input and output (and have an equals and hashCode override), accessing the semantic API constructs semantic objects with their read-out fields.
• For the classes in the semantic API that only occur on one side the readers are simply views on the syntax objects that forward the getters (and implement fallback behavior).
• For the classes in the semantic API that occur only on one side, the writers simply contain a syntax node and forward the writes (and implement fallback behavior).

One interesting case is the EncodedAsset because it intentionally "leaks" the underlying JSON encoding into the semantic API as an extension point.
(The extension point for configs doesn't have the same "leakage", because it's never passed from input to output, so there is no need to extra all data including keys one doesn't know about.)

Testing

This implementation passes all the existing test suites.

Open issues

This PR does not use the schemas yet for syntax checking. Unfortunately package:json_schema does not produce useful error messages. Moreover, introducing a concept of syntax errors next to semantic errors is a larger refactoring.

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

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pkgs/jni/lib/src/third_party/generated_bindings.dart
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[HosseinYousefi]

In general for all the throw UnimplementedError()s, add a description on what these conditions are and why they are not implemented for the untrained eye (like mine!)

[HosseinYousefi]

LGTM with small nits