fix(router): box the authentication authenticate future
The clippy gate fails on this branch with a 16544-byte future at the
`authentication_authenticate_core` call in `payment_external_authentication`,
over the 16384-byte `clippy::large_futures` threshold that `-D warnings` makes
an error.
The call itself is unchanged from main, and so is every line around it. What
changed is the size of what it awaits: the boundaries this branch declares carry
their observation state inside the futures they wrap, and that pushed this one
over a limit it was already sitting close to. So this is not a defect this
branch introduced in the call, it is a threshold this branch crossed.
Boxing moves the future to the heap, which is the remedy the lint names and the
one used elsewhere in this file for the same reason. It costs one allocation on
a path that is already making network calls.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
