Add typed JSON output to theme preview

[gonzaloriestra]

WHY are these changes introduced?

theme preview already emits JSON, but its result has no discoverable schema and its service also renders output and opens the browser.

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

WHAT is this pull request doing?

Return a typed preview result and move presentation and browser opening into the command. Preserve the compact JSON keys, success messages, file/API errors, and nonfatal browser behavior. Browser failures use typed diagnostics in JSON mode.

Normal output:

Preview is ready
• https://abc123.shopifypreview.com
• Preview ID: abc123

Matching result:

{"url":"https://abc123.shopifypreview.com","preview_identifier":"abc123"}

Compatibility decision: this draft preserves the existing stderr channel. During the full command lifecycle, the existing event context wraps that JSON string in an info diagnostic's message; stdout stays empty. Moving the final result to stdout needs a separate compatibility decision. Tests capture both streams and cover the current behavior explicitly.

Refresh the manifest, README, and generated docs. This PR reuses the output-capture helper from #8639.

How to manually test your changes?

pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json
pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json --preview-id YOUR_PREVIEW --open
pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json --json
pnpm shopify theme preview --help
pnpm shopify theme preview --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.