[ffe-account-transfer] Step 1: Define FfeAccountTransferAdapter trait methods

Plan

docs/modules/ROOT/pages/plans/ffe-account-transfer.adoc - Step 1

Description

Implement Step 1 (Define FfeAccountTransferAdapter trait methods) as described in the plan.

Acceptance Criteria

• Implementation matches plan specification
• Unit tests pass
• Integration tests pass (if applicable)
• cargo clippy --workspace -- -D warnings clean
• cargo fmt --check clean