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
Introduces a new v2 JSON-RPC surface (including exportAccount) across multiple packages, so any mismatch between request/response validation and Snap implementations could break integrations. Changes are mostly additive, but they touch request dispatching and account export flows which are sensitive to correctness.

Overview
Adds keyring v2 JSON-RPC support end-to-end: new KeyringRpcV2Method enum plus typed request/response superstructs in keyring-api, a new KeyringClientV2 in keyring-snap-client that sends/validates v2 RPC calls, and a new KeyringInternalSnapClientV2 wrapper for communicating with Snaps via the controller messenger.

Adds keyring-snap-sdk support via handleKeyringRequestV2, which validates v2 requests and dispatches them to KeyringV2 methods (with explicit handling for optional exportAccount). Also tightens isKeyringRpcMethod to be a TS type predicate and includes new unit tests and changelog updates.

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