Bug: TypeError in getSelectedMultichainAccount when account.scopes is undefined (Solana network)

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## Description

During controller initialization in `setupControllerEventSubscriptions()`, calling `getSelectedMultichainAccount` for the Solana network (`solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp`) throws a `TypeError` because `account.scopes` is `undefined` for at least one account, causing `isScopeEqualToAny` to fail when it tries to call `.some()` on an undefined value.

**Sentry Issue**: [METAMASK-YHAH](https://metamask.sentry.io/issues/7239174035/)

## Error

```
Error: Failed to get selected multichain account for network: solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp
Caused by: TypeError: can't access property "some", t is undefined
```

## Stack Trace

```
app/scripts/metamask-controller.js — setupControllerEventSubscriptions
NON_EVM_ACCOUNT_CHANGED_CONFIGS.forEach(({ network }) => { ... })

@metamask/accounts-controller — AccountsController.getSelectedMultichainAccount
const accounts = this.listMultichainAccounts(chainId);

@metamask/accounts-controller — AccountsController.listMultichainAccounts
return accounts.filter((account) => isScopeEqualToAny(chainId, account.scopes));

@metamask/keyring-utils — isScopeEqualToAny
return scopes.some((other) => isScopeEqual(scope, other));
// ^^^ `scopes` (i.e. account.scopes) is undefined
```

## Root Cause

`AccountsController.listMultichainAccounts` filters internal accounts by calling `isScopeEqualToAny(chainId, account.scopes)`. The `isScopeEqualToAny` function in `@metamask/keyring-utils` calls `scopes.some(...)` without guarding against `scopes` being `undefined`. If any internal account has an undefined `scopes` field (e.g. a corrupted or partially-migrated account), this throws a `TypeError`.

The error is caught in `setupControllerEventSubscriptions()` (~line 1841) and sent to Sentry, so it does not crash the extension — but it indicates a data integrity issue and prevents proper initialization of `lastSelectedAccountAddressByNetwork` for Solana.

## Relevant Code

`app/scripts/metamask-controller.js` (~line 1832):
```js
NON_EVM_ACCOUNT_CHANGED_CONFIGS.forEach(({ network }) => {
try {
lastSelectedAccountAddressByNetwork[network] =
this.accountsController.getSelectedMultichainAccount(network)?.address;
} catch (err) {
// This scenario shouldn't occur, but if it does, we track it for debugging
const error = new Error(
`Failed to get selected multichain account for network: ${network}`,
{ cause: err },
);
captureException(error);
}
});
```

## Environment

- **Extension version**: 13.20.1
- **Manifest**: MV2
- **Browser**: Firefox 149.0
- **OS**: Windows ≥10
- **Environment**: Production

## Possible Fix (AI suggested)

The fix could be applied at one or more levels:
1. **`@metamask/keyring-utils` — `isScopeEqualToAny`**: Guard against `scopes` being `undefined`/`null` (e.g. `return Array.isArray(scopes) && scopes.some(...)`).
2. **`@metamask/accounts-controller` — `listMultichainAccounts`**: Filter out accounts with undefined/empty `scopes` before calling `isScopeEqualToAny`.
3. **Migration**: Add a state migration to ensure all existing internal accounts have a valid (possibly empty) `scopes` array.

Start in app/scripts/metamask-controller.js at setupControllerEventSubscriptions(), then trace AccountsController.getSelectedMultichainAccount and listMultichainAccounts into isScopeEqualToAny in @metamask/keyring-utils. Determine where undefined account.scopes should be handled, including whether existing accounts need migration. Done means Solana initialization no longer throws or reports this Sentry error when an account lacks scopes.

Written by the indexing model from the issue text.