related tf-migrate PR: cloudflare/tf-migrate#125

What changed:
Added provider-side state migration infrastructure for 3 resources transitioning from SDKv2 (v4) to Plugin Framework (v5).

Resources migrated:

• 🔄 dns_record - renamed resource + state transformations
• 🔄 page_rule - complex JSON transformations
• 🔄 load_balancer_pool - nested object transformations

New infrastructure (per resource): migration/v500/
├── handler.go - StateUpgrader functions
├── model.go - Source (v4) and target (v5) models
├── schema.go - v4 schema definition for parsing old state
├── transform.go - State transformation logic
├── migrations_test.go - Migration tests (v4→v5)
└── testdata/ - Test configs (v4 + v5)

Key features:

• ✅ Controlled rollout: TF_MIG_TEST=1 enables migrations during development
• ✅ Automatic migration: StateUpgraders run during terraform apply (no manual steps)
• ✅ Comprehensive tests: Dual test cases validate both v4→v5 and v5→v5 upgrades
• ✅ Moved old tests: Deleted 1,448 lines of old migration tests (consolidated into new structure)

Notable transformations:

• load_balancer_pool: SDK v2 TypeList MaxItems:1 (arrays) → Plugin Framework SingleNestedAttribute (objects)
• dns_record: Resource rename + MoveState implementation
• page_rule: Complex nested JSON structure changes

Result: +3,687 lines of migration infrastructure that enables seamless v4→v5 upgrades for users.