contracts/deposit: cache parsed ABI and avoid per-call parsing

[GarmashAlex]

Removed unnecessary allocations and repeated ABI parsing in the hot path that processes deposit logs. Previously, UnpackDepositLogData converted the constant ABI string to a []byte, wrapped it in a bytes.Reader, and reparsed the ABI JSON on every call before unpacking into a []interface{}, incurring repeated allocations and type assertions. We now cache a parsed abi.ABI at package level using strings.NewReader and use UnpackIntoInterface to populate the generated DepositContractDepositEvent struct directly, eliminating the interface slice and assertions. The ABI is immutable and abi.ABI is safe for concurrent read-only reuse, so the cached instance is thread-safe. Behavior remains identical while reducing CPU and GC pressure during log processing.