refactor(a2a): name the fail-closed dispatch predicate once

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.

plugins/platforms/a2a/adapter.py

@@ -331,7 +331,7 @@ class A2AAdapter(BasePlatformAdapter):
         logger.info("A2A: serving Agent Card + JSON-RPC on http://%s:%s (%s) as %r; %d routed agent(s)", self.host, self.port,
                     "localhost-only" if self._security_context.localhost_only() else "REMOTE (bearer auth)", self.agent_name, len(self._agents))
         sec = self._security_context
-        if not (sec.allow_all_users or sec.localhost_only() or sec.trusted_peers or sec.is_loopback_bind()):
+        if sec.dispatch_fails_closed():
             logger.error("A2A: exposed on non-loopback bind %s with no A2A_TRUSTED_PEERS; every dispatch will be refused (403). "
                          "Set A2A_TRUSTED_PEERS, or A2A_ALLOW_ALL_USERS=true for a trusted network.", self.host)
         self._wire_plugin_handlers(None)  # plugin-registered native handlers

plugins/platforms/a2a/security.py

@@ -16,6 +16,7 @@ import time
 import urllib.parse
 from dataclasses import dataclass
 from typing import Optional
+from agent.proxy_bypass import is_loopback_host
 from gateway.platforms._shared import profile_scoped as _profile_scoped
 
 logger = logging.getLogger(__name__)
@@ -100,18 +101,18 @@ class A2ASecurityContext:
         return None
 
     def is_loopback_bind(self) -> bool:
-        """True when the resolved bind host is loopback."""
-        from agent.proxy_bypass import is_loopback_host
         return is_loopback_host(self.resolve_bind_host())
 
+    def dispatch_fails_closed(self) -> bool:
+        """Network-exposed token bind with no allow-list and no allow-all: refuse every peer."""
+        return not (self.allow_all_users or self.localhost_only() or self.trusted_peers or self.is_loopback_bind())
+
     def is_trusted_peer(self, identity: str) -> bool:
         """Fail closed on network-exposed binds with no allow-list; loopback
         binds without an allow-list stay open for backward compatibility."""
-        if self.allow_all_users or self.localhost_only():
-            return True
-        if not self.trusted_peers:
-            return self.is_loopback_bind()  # the misconfiguration is logged once in A2AAdapter.connect()
-        return identity in self.trusted_peers
+        if self.dispatch_fails_closed():
+            return False  # the misconfiguration is logged once in A2AAdapter.connect()
+        return self.allow_all_users or self.localhost_only() or not self.trusted_peers or identity in self.trusted_peers
 
     def sign_push_payload(self, payload: dict) -> str:
         """HMAC-SHA256 hex over the sorted-key JSON body; "" when no secret."""