feat: add KeyringClientV2 support

[ccharly]

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

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Note

Introduces keyring v2 JSON-RPC schema and adds v2 clients and SDK request handler, with comprehensive tests and exports.

• API (packages/keyring-api)
  • RPC v2 schema: Add keyring-rpc.ts with KeyringRpcV2 types, KeyringRpcV2Method enum, type guard isKeyringRpcV2Method, and request/response structs for getAccounts, getAccount, createAccounts, deleteAccount, exportAccount, submitRequest.
  • Exports: Re-export ./keyring-rpc in api/v2/index.ts.
  • Type guard: Make isKeyringRpcMethod a type predicate; fix test description string.
  • Tests: Add keyring-rpc.test.ts for v2 method guard.
• Snap Client (packages/keyring-snap-client)
  • New KeyringClientV2: Implements v2 methods; validates responses via structs; adds tests.
  • Exports: Export v2 from src/index.ts and v2/index.ts.
• Internal Snap Client (packages/keyring-internal-snap-client)
  • New KeyringInternalSnapClientV2: Extends KeyringClientV2, sends via SnapControllerMessengerSender; supports withSnapId; tests added.
  • Sender: Export SnapControllerMessengerSender.
  • Exports: Re-export v2 from index.ts and v2/index.ts.
• Snap SDK (packages/keyring-snap-sdk)
  • RPC handler v2: Add handleKeyringRequestV2 to dispatch/validate v2 requests; throws MethodNotSupportedError when appropriate; tests added.
  • Exports: Export v2 handler from index.ts and v2/index.ts.

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