Add typed JSON output to theme open

[gonzaloriestra]

WHY are these changes introduced?

theme open only prints links for humans. Scripts need a typed, discoverable result containing the selected theme and its URLs.

Part of https://github.com/shop/issues-develop/issues/23695.

WHAT is this pull request doing?

Add --json and a domain-owned result schema. Keep theme selection, terminal text, and browser opening behavior unchanged, with separate execution and presentation.

Normal output:

Preview information for theme 'My theme' (#1)
• Preview your theme
• Customize your theme at the theme editor

JSON output:

{
  "theme": {
    "id": 1,
    "name": "My theme",
    "createdAtRuntime": false,
    "processing": false,
    "role": "unpublished"
  },
  "preview_url": "https://store.myshopify.com?preview_theme_id=1",
  "editor_url": "https://store.myshopify.com/admin/themes/1/editor"
}

The optional theme.src field is included when available. Refresh the command manifest, README, and generated documentation. Add service, schema, presentation, and real stdout/stderr tests.

How to manually test your changes?

pnpm shopify theme open --store YOUR_STORE --theme YOUR_THEME
pnpm shopify theme open --store YOUR_STORE --theme YOUR_THEME --editor
pnpm shopify theme open --store YOUR_STORE --theme YOUR_THEME --json
pnpm shopify theme open --help
pnpm shopify theme open --json-schema

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
• A single changeset for all theme migrations is added in the last PR, #8643.