Test helpers for WebAuthn component tests.

These helpers build a mock AshAuthentication.Strategy.WebAuthn struct used only for rendering tests (gated by the :webauthn_strategy_required tag).

The mock uses Example.Accounts.User as its :resource because the component only needs the resource for subject-name derivation via Info. Actual WebAuthn ceremony tests (credential creation, sign-in) are not covered here — those belong in the upstream ash_authentication package alongside a dedicated fixture like Example.UserWithWebAuthn.