fix(mcp): let link_list return kernel_datapath

The tool descriptions have always advertised kernel_datapath as a
requestable field, but the handler's whitelist did not contain it: asking
for it was rejected as an invalid field, so the MCP clients could not see
which links run on kernel interfaces at all.

Also record what seeding a netem experiment on IOU costs: IOL emits its own
CDP/keepalive frames and they draw from the netem RNG stream, so a
seeded run only reproduces after `no cdp run` (verified bit-identical pcaps).

docs/features/iou-kernel-datapath.md

@@ -152,6 +152,12 @@ Harmless on the relay, where suspend is emulated by the synthetic
 frequency_drop filter, but fatal for a kernel link, whose suspend *is* the
 anchor's admin state — a "suspended" link kept forwarding.
 
+**Measuring seeded impairments on IOU**: IOL emits CDP/keepalive frames of
+its own, and they draw from the netem RNG stream — a `seed`-reproducible run
+never matches while they are on (measured: three rounds of mismatches, then
+bit-identical pcaps after `no cdp run`). Turn the platform chatter off before
+comparing seeded runs; the extra frames are visible in the capture.
+
 ## Configuration
 
 ```ini

gns3server/agent/gns3_copilot/gns3_client/api_handlers.py

@@ -105,6 +105,10 @@ VALID_LINK_FIELDS = {
     "link_id",
     "project_id",
     "link_type",
+    # Runtime datapath fact (kernel bridge vs uBridge relay): the MCP tool
+    # descriptions have always advertised it, but it was missing here, so
+    # link_list could not return it at all.
+    "kernel_datapath",
     "nodes",
     "suspend",
     "link_style",

tests/agent/mcp/test_handlers.py

@@ -425,6 +425,21 @@ class TestLink:
             result = get_links_handler({"project_id": "p1", "fields": ["link_id"]}, ctx)
             assert result["links"] == [{"link_id": "l1"}]
 
+    def test_list_can_ask_for_kernel_datapath(self, ctx):
+        """
+        The datapath is a field the tool descriptions always advertised, so
+        it has to be requestable: a whitelist missing it rejected
+        fields=["kernel_datapath"] and made link_list unable to report which
+        links run on kernel interfaces.
+        """
+
+        from gns3server.agent.gns3_copilot.gns3_client.api_handlers import get_links_handler
+
+        with patch(f"{AH}._get_connector") as m:
+            m.return_value = _mock_conn([{"link_id": "l1", "link_type": "ethernet", "kernel_datapath": True}])
+            result = get_links_handler({"project_id": "p1", "fields": ["link_id", "kernel_datapath"]}, ctx)
+            assert result["links"] == [{"link_id": "l1", "kernel_datapath": True}]
+
     def test_get(self, ctx):
         from gns3server.agent.gns3_copilot.gns3_client.api_handlers import get_link_handler