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.

Normal output includes operation-one: COMPLETED (0/1 settled). The matching JSON (shown compactly) is:

{"schemaVersion":1,"operations":[{"id":"operation-one","status":"COMPLETED","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