fix(keyring-snap-sdk): use KeyringRpc

[ccharly]

Use KeyringRpc instead of Keyring (v2) type, since the Snap does not need to implement the full Keyring (v2) interface, only a subset of it (KeyringRpc, just the RPC-callable methods basically)

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Note

Low Risk
Type-only API tightening in the Snap SDK with no change to RPC dispatch logic or security-sensitive paths.

Overview
handleKeyringRequest (v2) now types its keyring argument as KeyringRpc instead of the full v2 Keyring, matching what Snap onKeyringRequest handlers actually need to implement (RPC-callable methods only, not client-side concerns like serialize / deserialize / type / capabilities).

Tests and the Unreleased changelog are updated accordingly; runtime dispatch behavior is unchanged.

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