feat(keyring-snap-bridge): add isSnapKeyring

[ccharly]

A small helper to check and narrow a Keyring (v2) type to SnapKeyring (v2).

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Note

Low Risk
Low risk: introduces a pure type-guard helper (keyring.type === KeyringType.Snap) plus unit tests and a changelog entry, without altering keyring behavior.

Overview
Adds isSnapKeyring to the v2 SnapKeyring module as a type-guard helper to detect Snap keyrings by type and narrow Keyring to SnapKeyring.

Extends SnapKeyring.test.ts with coverage for true/false cases and type narrowing, and documents the addition in CHANGELOG.md.

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