fix(host_agent): maintenance reads the swarm through the loaded Docker plugin, not by importing its models again (#5768)

Importing plugins.docker.models by path defines its tables a second time,
because the plugin manager loaded them as _steward_plugin_docker.models.
Inside the app that raises InvalidRequestError, not ImportError. The cadence
integration test hit it, and #5767's _swarm (used when a job is planned)
carried the same import. Both now go through maintenance.docker_models().

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

plugins/host_agent/cadence.py

@@ -172,11 +172,9 @@ async def _back(session, host, job, reg, now: datetime) -> str | None:
     node = None
     services: list = []
     if job.node_id:
-        try:
-            from plugins.docker.models import DockerSwarmNode, DockerSwarmService
-        except ImportError:
-            DockerSwarmNode = DockerSwarmService = None
-        if DockerSwarmNode is not None:
+        models = maintenance.docker_models()
+        if models is not None:
+            DockerSwarmNode, DockerSwarmService = models.DockerSwarmNode, models.DockerSwarmService
             node = (await session.execute(select(DockerSwarmNode).where(
                 DockerSwarmNode.node_id == job.node_id)
                 .order_by(DockerSwarmNode.updated_at.desc()).limit(1))).scalar_one_or_none()