runtime: handle GRC servers without XML-RPC introspection

GRC's SimpleXMLRPCServer uses register_instance() which doesn't
expose system.listMethods. Wrap the connectivity check in a
try/except so a Fault is treated as "connected" while
ConnectionRefusedError still propagates.

src/gnuradio_mcp/middlewares/thrift.py

@@ -235,7 +235,9 @@ class ThriftMiddleware:
                     units=prop.units if hasattr(prop, "units") else None,
                     min_value=prop.min.value if prop.min else None,
                     max_value=prop.max.value if prop.max else None,
-                    default_value=prop.defaultvalue.value if prop.defaultvalue else None,
+                    default_value=(
+                        prop.defaultvalue.value if prop.defaultvalue else None
+                    ),
                     knob_type=knob_type,
                 )
             )
@@ -287,9 +289,7 @@ class ThriftMiddleware:
                     avg_work_time_us=metrics.get("avg work time", 0.0),
                     total_work_time_us=metrics.get("total work time", 0.0),
                     avg_nproduced=metrics.get("avg nproduced", 0.0),
-                    input_buffer_pct=self._to_list(
-                        metrics.get("avg input % full", [])
-                    ),
+                    input_buffer_pct=self._to_list(metrics.get("avg input % full", [])),
                     output_buffer_pct=self._to_list(
                         metrics.get("avg output % full", [])
                     ),

src/gnuradio_mcp/middlewares/xmlrpc.py

@@ -31,8 +31,16 @@ class XmlRpcMiddleware:
         transport = xmlrpc.client.Transport()
         transport.timeout = XMLRPC_TIMEOUT
         proxy = xmlrpc.client.ServerProxy(url, transport=transport)
-        # Verify connectivity
-        proxy.system.listMethods()
+        # Verify connectivity — GRC's SimpleXMLRPCServer uses
+        # register_instance() which doesn't enable system.listMethods.
+        # A Fault means the server responded (connected); only network
+        # errors like ConnectionRefused indicate no server.
+        try:
+            proxy.system.listMethods()
+        except ConnectionRefusedError:
+            raise  # actual connectivity failure — propagate
+        except Exception as e:
+            logger.debug("Introspection unavailable (normal for GRC servers): %s", e)
         logger.info("Connected to XML-RPC at %s", url)
         return cls(proxy, url)
 

tests/unit/test_thrift_middleware.py

@@ -125,7 +125,9 @@ class TestThriftMiddlewareVariables:
         """list_variables excludes performance counter knobs."""
         mock_client.getKnobs.return_value = {
             "sig_source0::frequency": MockKnob("sig_source0::frequency", 1e6, 5),
-            "null_sink0::avg throughput": MockKnob("null_sink0::avg throughput", 1e9, 5),
+            "null_sink0::avg throughput": MockKnob(
+                "null_sink0::avg throughput", 1e9, 5
+            ),
         }
 
         variables = thrift_middleware.list_variables()

tests/unit/test_xmlrpc_middleware.py

@@ -53,6 +53,21 @@ class TestConnect:
             with pytest.raises(ConnectionRefusedError):
                 XmlRpcMiddleware.connect("http://localhost:8080")
 
+    def test_connect_without_introspection(self):
+        """GRC servers don't enable system.listMethods — connect should still succeed."""
+        from xmlrpc.client import Fault
+
+        with patch("gnuradio_mcp.middlewares.xmlrpc.xmlrpc.client") as mock_xmlrpc:
+            mock_proxy = MagicMock()
+            mock_proxy.system.listMethods.side_effect = Fault(
+                1, "method 'system.listMethods' is not supported"
+            )
+            mock_xmlrpc.ServerProxy.return_value = mock_proxy
+            mock_xmlrpc.Transport.return_value = MagicMock()
+
+            mw = XmlRpcMiddleware.connect("http://localhost:8080")
+            assert mw is not None
+
 
 class TestConnectionInfo:
     def test_get_connection_info(self, xmlrpc_mw, mock_proxy):