feat(swarm): services short, nodes down and managers lost are alertable (#5788)

On 2026-10-14 nine swarm services sat at 0/1 and nothing fired. From each
manager's swarm report the docker plugin now records three figures on
resource "swarm": swarm_services_short, swarm_nodes_down and
swarm_managers_unreachable. Drained nodes are left out of the last two,
since a drain is deliberate. The figures are recorded at most once every
30 s whichever manager reported, and are offered to alert rules.

Agent 1.19.0 sends each manager's ManagerStatus.Reachability. Older agents
fall back to the node's status.

A maintenance job on a swarm node also keeps a window over docker/swarm,
so its drain and reboot hold those alerts until the job ends.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/host_agent/test_agent_docker.py

@@ -298,16 +298,18 @@ def test_swarm_nodes_normalises_fields():
     nodes = [
         {"ID": "n1", "Description": {"Hostname": "mgr"},
          "Spec": {"Role": "manager", "Availability": "active"},
-         "Status": {"State": "ready"}, "ManagerStatus": {"Leader": True}},
+         "Status": {"State": "ready"}, "ManagerStatus": {"Leader": True, "Reachability": "Reachable"}},
         {"ID": "n2", "Description": {"Hostname": "wrk"},
          "Spec": {"Role": "worker", "Availability": "drain"},
          "Status": {"State": "down"}},
     ]
     out = {n["node_id"]: n for n in a._swarm_nodes(nodes)}
     assert out["n1"] == {"node_id": "n1", "hostname": "mgr", "role": "manager",
-                         "availability": "active", "status": "ready", "leader": True}
+                         "availability": "active", "status": "ready", "leader": True,
+                         "reachability": "reachable"}
     assert out["n2"] == {"node_id": "n2", "hostname": "wrk", "role": "worker",
-                         "availability": "drain", "status": "down", "leader": False}
+                         "availability": "drain", "status": "down", "leader": False,
+                         "reachability": ""}
 
 
 def test_collect_swarm_skips_on_worker(monkeypatch):