feat(money-account-controller): multi-keyring accounts and default (POC)

[matthiasgeihs]

Summary

• POC: MoneyAccountController can hold multiple KeyringAccounts from Money and MPC keyrings at once.
• Adds createMoneyAccount({ keyringType, ... }) (Money: create-if-missing; MPC: existing keyring only + addAccounts(1)), addMoneyAccount, setDefaultMoneyAccount, and persisted defaultMoneyAccountId.
• Breaking: MoneyAccount is now KeyringAccount; getMoneyAccount() with no args returns the default account, not the primary HD entropy source.

Test plan

• yarn workspace @metamask/money-account-controller run test
• Confirm Money create path still mints/reuses a Money Keyring account by entropy source
• Confirm MPC create path throws if no MPC keyring is registered, and requires keyringId when more than one exists
• Confirm first create/add sets default; later accounts do not override; setDefaultMoneyAccount switches
• Confirm addMoneyAccount registers an already-built account without KeyringController calls
• Confirm clearState clears accounts and default
• Follow-up: implement MPCKeyring.addAccounts in the accounts repo; do not treat this as client-ready until that lands

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