Breaking type change on v6

Previously supported and exposed type `EthErc4337Method` has been removed somewhere between v6.1.1 and v6.3.1, causing breakage.

https://github.com/MetaMask/core/actions/runs/9324657793/job/25670306481?pr=4355

```
Error: packages/accounts-controller/src/tests/mocks.ts(9,3): error TS2305: Module '"@metamask/keyring-api"' has no exported member 'EthErc4337Method'.
```