fix: surface non-empty vault import destination

[Guardiola31337]

Description

• WalletKit backup import currently reports a non-empty destination as a generic vault database error from the lower-level SQLite copy guard.
• For incident recovery flows, host apps need to distinguish this expected restore-state condition from fatal backup corruption or database failures without parsing the error message string.

Changes

• Add StorageError::VaultImportDestinationNotEmpty { table } for vault backup imports into a destination that already has rows.
• Preflight backup import destination tables before calling the lower-level plaintext copy guard.
• Update the backup import test to assert the typed error while preserving the existing data-integrity check.

Testing Instructions

• cargo fmt --all --check
• cargo test -p walletkit-core import_vault_backup

---

Note

Low Risk
Small, localized change to backup import error handling and an early guard; no changes to encryption, credential writes, or successful restore paths.

Overview
Vault backup import now returns StorageError::VaultImportDestinationNotEmpty { table } when the destination already has rows in backup tables (credential_records, blob_objects), instead of a generic vault DB error from the SQLite copy path.

CredentialVault::import_plaintext runs a preflight ensure_backup_destination_empty before cipher::import_plaintext_copy. Public import_vault_from_backup docs note this case. The non-empty-vault test asserts the typed error and still checks data is unchanged after a failed import.

^{Reviewed by Cursor Bugbot for commit b244bb6. Bugbot is set up for automated code reviews on this repo. Configure here.}

[Guardiola31337]

Related Android incident PR: worldcoin/wld-android#7895 handles INC-109 in the app while this WalletKit PR makes the non-empty credential-vault import destination explicit for consumers.

[kilianglas]

LGTM