WIP - fix(passkeys): pass challenge as Buffer to simplewebauthn to fix lookup

[dschom]

WIP

Because

• simplewebauthn v13 treats string challenges as UTF-8 text and re-encodes them to base64url, producing a different value than what was stored in Redis. This caused challenge lookup to fail on registration/authentication finish.

This pull request

• Passes challenge as Buffer.from(challenge, 'base64url') to generateRegistrationOptions and generateAuthenticationOptions so simplewebauthn base64url-encodes the raw bytes (roundtrip-safe).
• Fixes PasskeyService constructor args in the integration test (was missing PasskeyConfig parameter).
• Adds webauthn-adapter.in.spec.ts with a virtual authenticator that exercises real simplewebauthn verifyRegistrationResponse (no mocks).
• Adds happy-path integration test for registration start → finish.

Issue that this pull request solves

Closes: (issue number)

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