feat(account-api)!: use keyring capabilities + createAccounts options for AccountProvider

[ccharly]

Start using new keyring v2 idioms for the AccountProvider.

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Note

Medium Risk
This is a breaking public API type change; downstream providers/consumers must update implementations and call sites, but there is minimal runtime logic change beyond interface conformance.

Overview
Updates the AccountProvider API to align with keyring v2 by adding a required capabilities: KeyringCapabilities getter and changing createAccounts to accept CreateAccountOptions instead of { entropySource, groupIndex }.

Documents the change as BREAKING in the account-api changelog, noting the new options enable additional account construction modes (e.g., multiple indexes at once).

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[ccharly]

@metamaskbot publish-previews

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.