fix(client): default_gateway never raises

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uc9V1v7iXruubpoLiSE24M

tests/test_gateway.py

@@ -1,3 +1,6 @@
+import asyncio
+import struct
+
 from hbd.client import gateway
 
 PROC_V4 = (
@@ -62,3 +65,11 @@ def test_windows_v4_lowest_metric():
 
 def test_windows_v6():
     assert gateway.parse_windows_route(WIN_V6, "ipv6") == "fe80::1"
+
+
+def test_default_gateway_never_raises_on_parser_error(monkeypatch):
+    """default_gateway catches all exceptions; never raises even if parser fails."""
+    monkeypatch.setattr("sys.platform", "linux")
+    monkeypatch.setattr(gateway, "parse_proc_route_v4", lambda _: (_ for _ in ()).throw(struct.error))
+    result = asyncio.run(gateway.default_gateway("ipv4"))
+    assert result is None