connect() and is_trusted_peer() each spelled out the same
"network-exposed token bind with no allow-list" condition; drift between
the two would let the startup warning and the actual refusal disagree.
Extract A2ASecurityContext.dispatch_fails_closed() and use it in both
places (truth table unchanged). Also hoist the is_loopback_host import to
module top (no cycle) and drop the restating docstring.