feat: add KeyringClientV2 support

[ccharly]

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

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Note

Medium Risk
Adds a new v2 JSON-RPC surface (including exportAccount) plus new clients/dispatch logic, which could affect request routing and key export behavior if integrated incorrectly. Changes are mostly additive but touch security-adjacent account/export flows.

Overview
Introduces Keyring v2 JSON-RPC plumbing end-to-end. @metamask/keyring-api now exports KeyringRpcV2Method plus typed request/response structs for getAccounts, getAccount, createAccounts, deleteAccount, exportAccount, and submitRequest, along with an isKeyringRpcV2Method guard.

Adds a new KeyringClientV2 (and KeyringInternalSnapClientV2) that sends v2-formatted JSON-RPC requests and validates responses with structs, and adds handleKeyringRequestV2 in keyring-snap-sdk to dispatch v2 methods to a KeyringV2 implementation (including explicit unsupported-method errors for optional exportAccount).

Minor cleanup: exports the new v2 modules from package index.ts files, makes SnapControllerMessengerSender public, and tightens isKeyringRpcMethod to a TypeScript type guard (plus test message fix).

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[cursor]

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

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[cursor]

Missing isSnapError check wraps Snap errors as generic Error

High Severity

The handleKeyringRequestV2 catch block is missing the isSnapError(error) check that the v1 handleKeyringRequest has. The v2 handler throws MethodNotSupportedError (which extends MethodNotSupportedRpcError from @metamask/snaps-sdk) in two places, but these structured Snap errors get caught and re-thrown as plain Error, losing their RPC error code and serialization data. The v1 handler correctly preserves Snap errors by checking isSnapError(error) and re-throwing them before the generic wrapping.