Downgrade app access scopes from write to read

Summary

• Introduces appDowngradeAccessScopes mutation (API v2027-01) to replace optional write scopes with matching read scopes for Shopify‑managed installations.
• Only optional write scopes can be downgraded; required scopes remain unchanged and invalid requests are rejected without altering current scopes.
• Use this when the app no longer needs write access—revoking a write scope also removes its read access unless granted separately.