bug: mnemonics should never be converted to a string; `@metamask/eth-hd-keyring` converts it to a string in its `deserialize` method

[davidmurdoch]

Example call:

core/packages/keyring-controller/src/KeyringController.ts

Line 2434 in 2b8c61d

await keyring.deserialize(data);

Related issue: MetaMask/accounts#250

In the extension's metamask-controller.js's submitPassword function we call keyringController.submitPassword(password) which calls its own restoreSerializedKeyrings method. This eventually calls into @metamask/eth-hd-keyring's deserialize method, which ultimately converts the mnemonic to a string.

Here is a trace:

Related Slack conversation: https://consensys.slack.com/archives/C087GPTR5HQ/p1741974188846359?thread_ts=1741795570.164739&cid=C087GPTR5HQ