test: make vault listener notification test deterministic

[Guardiola31337]

Description

• Make the vault-changed listener tests wait for observed callback delivery instead of relying on a fixed sleep.
• Drain the store notification before asserting the delete notification, matching the production listener channel's best-effort/coalescing behavior.
• This keeps PR 417 release-only; once this lands, release-plz can include it automatically in a fresh release PR.

Testing

• cargo fmt --check
• cargo test -p walletkit-core storage::credential_storage::tests::test_vault_changed_listener --lib
• cargo test -p walletkit-core --lib
• cargo clippy -p walletkit-core --lib --tests -- -D warnings

Notes

• cargo test --workspace --all-features gets past the walletkit-core lib suite locally, then fails in walletkit-core/tests/authenticator_integration.rs because anvil is not installed locally (SpawnError, OS No such file or directory).