Add typed JSON output to app import channel-config - #8821

WHY are these changes introduced?

Give scripts a strict channel-config import result. Related to #23958.

WHAT is this pull request doing?

Separate the import result from terminal output. Return a success status, an absolute TOML path, the unchanged TOML, and export warnings. Validate the result before writing files, preserve text output and --force, and use the existing schema getter and warning helpers.

This is a major JSON change: use path directly instead of joining it to the app directory.

Text: Imported the channel spec for Example app.
JSON: {"status":"success","handle":"example","filename":"example.toml","path":"/tmp/app/extensions/channel-config/specifications/example.toml","toml":"handle = \"example\"\n","warnings":[]}.

How to manually test your changes?

Use a copy of a linked app that is allowlisted for channel-spec export.

1. Run shopify app import channel-config --path <app> --json --no-input > result.json 2> events.jsonl. Check the absolute path, compare the native toml with that file, and check warning events on stderr.
2. Repeat; expect one error and no overwrite. Add --force and check successful replacement. Run without --json to compare warning and success text.
3. Run shopify app import channel-config --json-schema to inspect the contract. Use a missing app path with --json --no-input; expect one error document and a nonzero exit.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• Added the required major changeset