Add JSON Schema for subscription migration scheduling

[gonzaloriestra]

WHY are these changes introduced?

Make the existing scheduling JSON contract discoverable and validated.

Related to shop/issues-develop#23886. Stacked on #8586.

WHAT is this pull request doing?

Add a shared submission schema using the operation and error schemas from #8541, and infer submission types from it. Connect the shared schedule/unschedule encoder while preserving accepted operations, failure details, watch output, and exit codes. Expose the contract on schedule; unschedule follows later in this stack.

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

Example output

shopify app subscription-migrations schedule --input migrations.csv --force

Text output (banner borders omitted):

Subscription migrations scheduled.

Shops: 1
Operation IDs:
operation-one
Save every operation ID. You will need them to check or cancel this submission.

With --json:

{
  "clientId": "client-id",
  "action": "schedule",
  "inputDigest": "input-digest",
  "total": 1,
  "operations": [
    {
      "batchIndex": 0,
      "batchPayloadDigest": "batch-digest",
      "operation": {
        "id": "operation-one",
        "status": "RUNNING",
        "total": 1,
        "results": {
          "edges": []
        }
      }
    }
  ]
}

How to manually test your changes?

shopify app subscription-migrations schedule --help
shopify app subscription-migrations schedule --json-schema
shopify app subscription-migrations schedule --input migrations.csv --force --json
shopify app subscription-migrations schedule --input migrations.csv --force --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