feat(agentnetwork): thread the zone config through to the manager

Adds AgentNetwork.Zone to the management config and passes it to the
agent-network manager, alongside the existing plumbing in the combined
binary. Nothing reads it yet -- the allocator that stamps it onto new rows
comes next -- so this commit is inert on its own.

proxy/internal/middleware/builtin/agentnetwork_chain_integration_test.go

@@ -53,7 +53,7 @@ func newChainIntegration(t *testing.T) *chainIntegrationFixture {
 	require.NoError(t, err)
 	t.Cleanup(cleanUp)
 
-	manager := agentnetwork.NewManager(st, nil, nil, nil)
+	manager := agentnetwork.NewManager(st, nil, nil, nil, "")
 
 	server := &mgmtgrpc.ProxyServiceServer{}
 	server.SetAgentNetworkLimitsService(manager)