fix(controller): re-arm window_drop and expose kernel_datapath

Three fixes from live-calibrating the kernel impairment filters on an
isolated instance (Docker-to-Docker veth link, per-sequence evidence):

- update_filters skipped an unchanged filters dict as a no-op, so a
  one-shot window_drop outage that had already passed could never be
  re-armed: re-applying the same values produced no datapath command at
  all. A PUT still carrying window_drop now reconciles, restarting the
  schedule from the apply as documented. Measured: identical re-apply
  moves the outage window from seq 50-70 to 70-91 (arm at +8s of the
  new apply).

- gemodel parameter semantics were documented backwards in the
  validation rules, available_filters, the MCP/copilot descriptions and
  the feature docs. Measured against the kernel (per-direction counters,
  offered = sent + dropped): p is the good-to-bad transition chance, r
  the bad-to-good transition, 1-h the loss chance in the bad state (the
  good state loses nothing) - mean one-way loss p/(p+r)*(1-h), e.g.
  [10,30,100] -> 24.8%/25.7% measured vs 25% predicted, [5,45,100] ->
  10.8% both ways vs 10%.

- kernel_datapath was silently stripped from every REST/MCP link payload
  by the response model (LinkBase carried no such field), so clients
  could not tell a kernel link from a relay one.

Also verified on the same setup: the 3-argument window_drop form works
exactly as specified ([3000,5000,100] -> a single seq 31-55 outage,
[20000,6000,100] -> seq 115-144), and both generations of the command
builder emit the parameters unpadded.

Adds an API test for the field and controller tests for the no-op skip
and the window_drop re-arm exemption.

tests/api/routes/controller/test_links.py

@@ -139,9 +139,36 @@ class TestLinkRoutes:
         assert mock.called
         link_id = response.json()["link_id"]
         assert response.json()["nodes"][0]["label"]["x"] == 42
+        # the runtime datapath fact survives the response model on both POST and GET
+        assert response.json()["kernel_datapath"] is False
         response = await client.get(app.url_path_for("get_link", project_id=project.id, link_id=link_id))
         assert response.status_code == status.HTTP_200_OK
         assert response.json()["nodes"][0]["label"]["x"] == 42
+        assert response.json()["kernel_datapath"] is False
+
+    async def test_get_link_kernel_datapath(
+        self, app: FastAPI, client: AsyncClient, project: Project, nodes: Tuple[Node, Node]
+    ) -> None:
+        """A kernel-wired link reports kernel_datapath true (was dropped by the schema)."""
+
+        node1, node2 = nodes
+        with asyncio_patch("gns3server.controller.udp_link.UDPLink.create"):
+            response = await client.post(
+                app.url_path_for("create_link", project_id=project.id),
+                json={
+                    "nodes": [
+                        {"node_id": node1.id, "adapter_number": 0, "port_number": 3},
+                        {"node_id": node2.id, "adapter_number": 2, "port_number": 4},
+                    ]
+                },
+            )
+
+        link_id = response.json()["link_id"]
+        link = project.get_link(link_id)
+        link._link_data = [{"type": "nio_bridge"}]
+        response = await client.get(app.url_path_for("get_link", project_id=project.id, link_id=link_id))
+        assert response.status_code == status.HTTP_200_OK
+        assert response.json()["kernel_datapath"] is True
 
     async def test_update_link_suspend(
         self, app: FastAPI, client: AsyncClient, project: Project, nodes: Tuple[Node, Node]