Align subscription migration failure reason types and schema

[kevin-johnson-shopify]

WHY are these changes introduced?

Subscription migration readbacks can report PLAN_REQUIREMENTS_NOT_MET and SHOP_FEATURES_UNVERIFIED, but the CLI's failure-reason type and documented known values omit them. Developers inspecting the JSON schema therefore see an incomplete list.

WHAT is this pull request doing?

Add both values to the internal type and the JSON schema description, and regenerate the command manifest and README. Keep upstream values as nullable strings so future API additions continue to pass through JSON and CSV output.

Validation: 61 existing migration tests passed, including API projection, JSON/CSV output and command integration with mocked services. The CLI build, app typecheck, changed-file ESLint and command snapshot check passed. The emitted --json-schema includes both values and retains its nullable string type. Live provider integration was not exercised.

How to manually test your changes?

Run pnpm shopify app subscription-migrations list --json-schema from the repository root. Inspect definitions.Result.properties.subscriptions.items.properties.lastFailureReason: its description should list both new values, and its type should remain ["string", "null"]. This command requires no linked app or API request.

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
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset