Function begin 

pub fn begin(
    store: &dyn ConsentStore,
    request_uri: String,
    did: String,
    client_id: String,
    granted_scope: ScopeSet,
    redirect_uri: String,
    state: Option<String>,
    code_challenge: String,
    code_challenge_method: String,
    client_auth_key: Option<ClientAuthKey>,
    dpop_jkt: DpopThumbprint,
) -> ConsentReference

Mints a pending consent, for authorize to hand back as the reference the agent’s confirming tool call must carry.

expires_at and consumed on the passed PendingConsent are ignored and overwritten by the store — callers construct one with any placeholder for both.