Begin passkey enrollment for a connected user (always 403)

This endpoint always returns 403 and cannot be made to work. The description here previously said platform apps should use it instead of POST /v1/auth/passkeys/register/begin, which was wrong and cost integrators a WebAuthn ceremony to discover.

A passkey enrolled this way is login-capable: it can be asserted through the public sign-in flow to mint a full, non-delegated user session, so an app that enrolled one would hold a credential stronger than the delegation boundary it operates under. The capability was removed for that reason and will not return.

A passkey is enrolled by the user, in their own session, via POST /v1/auth/passkeys/register/begin. For a connected user that means completing the claim flow on the connection first.

To use a passkey a connected user already has, see:

• GET /v1/platform/connections/{connectionId}/passkeys — whether the confirm step can be offered at all.
• POST /v1/platform/connections/{connectionId}/passkeys/tx-assert/begin and .../complete — ask that user to touch their authenticator over a specific digest.