fix: use configured BIP port for gateway client transport

The client transport was hardcoded to port 0, causing the OS to assign
a random ephemeral port for outgoing BACnet requests. This breaks
device communication when devices respond to the standard BACnet port
(47808) and the client is listening on a different random port.

Use `bip.port` from the gateway config, matching the server transport
which already used the configured port correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: chappo

crates/bacnet-gateway/src/builder.rs

@@ -131,7 +131,7 @@ impl GatewayBuilder {
         let db = server.database().clone();
 
         // Create client transport (also needs foreign device config for broadcast routing).
-        let mut client_transport = BipTransport::new(interface, 0, broadcast);
+        let mut client_transport = BipTransport::new(interface, bip.port, broadcast);
         if let Some(ref fdc) = fd_config {
             client_transport.register_as_foreign_device(fdc.clone());
         }