fix(host_agent): a node the operator keeps drained stays drained through its maintenance (#5777)

#5777 "Maintenance would make a node the operator keeps drained active after
its reboot". A swarm node already set to drain or pause, such as a manager
kept free of work, is no longer drained before its reboot or made active
after it. Its job only updates and reboots it, and back-in-service needs it
ready but not active.

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

plugins/host_agent/cadence.py

@@ -74,19 +74,21 @@ def is_due(plan, now: datetime) -> datetime | None:
     return slot
 
 
-def back_in_service(*, last_seen_at, node, services, job_finished_at, now: datetime) -> str | None:
+def back_in_service(*, last_seen_at, node, services, job_finished_at, now: datetime,
+                    expect_active: bool = True) -> str | None:
     """None when the host is back in service, else why not yet. `node` is the
     freshest swarm-node row for the job's node (None outside a swarm, or when
     the job has no node); `services` the freshest row per swarm service. Only
     rows reported after the job finished count, so a stale "all fine" from
-    before the reboot never passes."""
+    before the reboot never passes. `expect_active` is False for a node the
+    job didn't drain (the operator keeps it drained): it need only be ready."""
     if last_seen_at is None or now - last_seen_at > POSTING_WITHIN:
         return "its agent isn't posting yet"
     if node is None:
         return None
     if node.updated_at < job_finished_at:
         return "no swarm report since it came back"
-    if node.status != "ready" or node.availability != "active":
+    if node.status != "ready" or (expect_active and node.availability != "active"):
         return f"its swarm node is {node.status}/{node.availability}"
     fresh = [s for s in services if s.updated_at >= job_finished_at]
     if not fresh:
@@ -184,7 +186,8 @@ async def _back(session, host, job, reg, now: datetime) -> str | None:
                     freshest[s.service_name] = s
             services = list(freshest.values())
     return back_in_service(last_seen_at=reg.last_seen_at if reg else None, node=node,
-                           services=services, job_finished_at=job.finished_at or now, now=now)
+                           services=services, job_finished_at=job.finished_at or now, now=now,
+                           expect_active=bool(job.drain_host_id))
 
 
 async def advance(session, run, plan, now: datetime) -> str: