Migrate pre-partition arn:aws: ARNs in state persisted by cn/gov-region servers #2610

The ARN partition work (#2569 to #2591) made every ARN use the region's partition (aws-cn, aws-us-gov, aws-iso*). State persisted by a China/GovCloud-region server before those changes still holds arn:aws:... values, and responses keep returning them.

Current behavior

Snapshots load as-is (crates/fakecloud-server/src/main.rs, e.g. KMS: serde_json::from_slice + schema-version check). No *_SNAPSHOT_SCHEMA_VERSION was bumped by the partition changes and fakecloud-persistence has no partition logic. Lookups still work because parsers (arn_resource, parse_kms_arn) accept any partition, but stored fields like KmsKey.arn and KmsAlias.alias_arn come back with arn:aws:, mixed with newly created arn:aws-cn: resources in the same responses.

Expected

A load-time migration that rewrites stored ARNs whose region belongs to a non-aws partition to that partition (per service snapshot, with a schema-version bump), including ARN-keyed maps (e.g. SNS topics). Scope: servers started with a cn/gov/iso --region against a pre-existing --data-path. Low priority since it only affects persisted state from before the change.