Add JSON Schema for subscription migration status

[gonzaloriestra]

WHY are these changes introduced?

Make the existing operation-status JSON contract discoverable and validated.

Related to shop/issues-develop#23886. Stacked on #8587.

WHAT is this pull request doing?

Reuse the cancellation operation schema for status output and infer operation types from the shared contract. Preserve operation order, per-shop results, terminal text, and exit behavior. Verify that watched progress reaches stderr as typed events while stdout contains one final JSON document.

The JSON contract follows CLI versioning and has no separate schemaVersion field.

Example output

shopify app subscription-migrations status --id operation-one

Text output (banner borders omitted):

Subscription migration operations.

operation-one: RUNNING (0/1 settled)

With --json:

{
  "operations": [
    {
      "id": "operation-one",
      "status": "RUNNING",
      "total": 1,
      "results": {
        "edges": []
      }
    }
  ]
}

How to manually test your changes?

shopify app subscription-migrations status --help
shopify app subscription-migrations status --json-schema
shopify app subscription-migrations status --id <operation-id> --json
shopify app subscription-migrations status --id <operation-id> --watch --json

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