Add typed JSON output to doc fetch

[isaacroldan]

WHY are these changes introduced?

doc fetch prints Markdown or saves a file, but scripts cannot request a typed result or discover its schema.

WHAT is this pull request doing?

Add --json and schema discovery. Keep document retrieval separate from presentation, preserve the default Markdown output, and return a file receipt after a successful write. The output file still contains the original Markdown.

Both examples show the same document excerpt; the remaining Markdown is omitted.

Default output:

---
title: Shopify CLI

JSON output:

{
  "document": {
    "url": "https://shopify.dev/docs/api/shopify-cli",
    "content": "---\ntitle: Shopify CLI\n"
  }
}

Validation: 36 focused tests, build, type-check, and lint pass. Live command checks and adversarial review cover stdout/stderr, file bytes and receipts, schema discovery, and failure exits.

How to manually test your changes?

pnpm shopify doc fetch --url https://shopify.dev/docs/api/shopify-cli
pnpm shopify doc fetch --url https://shopify.dev/docs/api/shopify-cli --json
pnpm shopify doc fetch --url https://shopify.dev/docs/api/shopify-cli --output ./shopify-cli.md --json
pnpm shopify doc fetch --json-schema

Compare the inline Markdown with the JSON document. For file output, check the receipt on stdout and the Markdown in shopify-cli.md.

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 a minor changeset for the new JSON flag.

[isaacroldan]

/snapit

[Suleimanlatrsh]

Tophatted locally, default / --json / --output / --language all look good and the saved file matches the JSON content exactly. Nice split between fetching and printing, much easier to follow. Couple things:

• needs a rebase, conflicts with #8790 in json-output-command-exceptions.js
• not from this PR, but --output docs/ (a folder) comes back as type: "bug" with a stack trace instead of an abort. Same if the parent is a file. The write-failure test only checks for error, so it'd be nice to assert the type there. Fine as a follow-up

[Suleimanlatrsh]

Q: shopify.dev redirects a lot (/docs/api/admin lands on /docs/api/admin-graphql) and this returns the URL that was passed in, not where we ended up. Would response.url be more useful here so scripts get the real page? Fine either way since the schema says "requested" URL