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.

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:

{
  "schemaVersion": 1,
  "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