Used by the login and consent application. Requires the signed-in user's Wingspan session token as a bearer token and an Origin header from the consent application (403 otherwise). Returns the client details, the requested resource, the Accounts and Roles the user can authorize for the interaction identified by interactionId, whether consent is required, and a single-use decisionNonce that must be sent to POST /v3/platform/oauth/authorize-decision. Interactions started with prompt=none are rejected; use POST /v3/platform/oauth/authorize-silent-complete for those. Returns 409 if the client's policy changed since the interaction began. The response is not cacheable.