If the session cannot be found, this call silently does nothing rather than throwing, since marking an already-gone session complete is harmless.
Call this from your own success-page logic or webhook handler once you've independently confirmed the payment or subscription succeeded. StripeKit does not call this automatically anywhere, including in $kit->webhooks->process().
After this call, submitFields() will reject further submissions for the session, since it requires status === 'open'.