feat: add KeyringClientV2 support

[ccharly]

Similar implementation than KeyringClient but using the new unified keyring API (keyring v2) methods.

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Note

Adds first-class support for the unified keyring v2 RPC surface across API, clients, and SDK.

• New keyring-api v2 RPC: KeyringRpcV2Method, request/response structs (getAccount(s), createAccounts, deleteAccount, exportAccount, submitRequest), and isKeyringRpcV2Method
• New keyring-snap-client KeyringClientV2 implementing v2 methods with strict response validation and export from v2/index
• New keyring-internal-snap-client KeyringInternalSnapClientV2 using SnapControllerMessengerSender, supports withSnapId, and exports
• New keyring-snap-sdk handleKeyringRequestV2 to validate/dispatch v2 JSON-RPC requests, throwing MethodNotSupportedError when appropriate
• Extensive tests for v2 API, clients, and handler; minor fix: isKeyringRpcMethod is now a type guard and test message corrected

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