Add JSON Schema for subscription migration listing

[gonzaloriestra]

WHY are these changes introduced?

Make the existing subscription migration list JSON contract discoverable and validated.

Related to shop/issues-develop#23886. Stacked on #8541.

WHAT is this pull request doing?

Add a domain-owned schema and infer the subscription type from it. Preserve streamed CSV, the versioned JSON envelope, nullable fields, and page-failure behavior.

Example output

shopify app subscription-migrations list

Text output for an empty list (CSV):

shop_id,status,manual_subscription_name,manual_subscription_price_amount,manual_subscription_price_currency_code,manual_subscription_interval,target_plan_handle,notification_kind,notification_opt_out_deadline,notification_sent_at,price_behavior,effective_date,last_failure_reason

With --json:

{
  "schemaVersion": 1,
  "subscriptions": []
}

How to manually test your changes?

shopify app subscription-migrations list --help
shopify app subscription-migrations list --json-schema
shopify app subscription-migrations list
shopify app subscription-migrations list --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