feat: v4 to v5 migration for regional_hostname

[rotem-cloud]

Add migration support for cloudflare_regional_hostname from provider v4
(SDKv2) to v5 (Plugin Framework).

Changes

• Add routing field with default "dns" to state
• Remove timeouts field from state and timeouts block from config
• Set schema_version to 0
• Preserve all user fields (zone_id, hostname, region_key)

Tests

Unit tests (16 tests, 75.0% coverage):

• 7 config transformation tests (basic, count, for_each, timeouts removal)
• 8 state transformation tests (routing addition, timeouts removal)
• 1 edge case test (empty state)

Integration tests (14 resource instances):

• All Terraform patterns: basic, count, for_each (map/set), conditional
• Wildcard hostnames, functions, variables
• Verifies zero data loss and correct field transformations

All tests passing ✓

Run tests:

go test ./internal/resources/regional_hostname/
TEST_RESOURCE=regional_hostname go test -v ./integration/v4_to_v5/