fix: narrow exception handling in list_nodes, move mock imports to top

- Remove redundant httpx.ConnectError from nodes except clause (it's a
  subclass of HTTPError so the tuple catch was redundant)
- Narrow services except clause from bare Exception to httpx.HTTPError,
  add logger.warning with coordinator_url for debuggability
- Move `from unittest.mock import MagicMock, patch` from mid-file to
  the top-of-file import block with the other stdlib/third-party imports

app/nodes.py

@@ -117,7 +117,7 @@ def list_nodes() -> list:
         r = httpx.get(f"{coordinator_url}/api/nodes", timeout=5.0)
         r.raise_for_status()
         coord_nodes: list[dict] = r.json()
-    except (httpx.HTTPError, httpx.ConnectError) as exc:
+    except httpx.HTTPError as exc:
         logger.warning("Coordinator unreachable: %s", exc)
         return []
 
@@ -125,7 +125,8 @@ def list_nodes() -> list:
         sr = httpx.get(f"{coordinator_url}/api/services", timeout=5.0)
         sr.raise_for_status()
         services_data: list[dict] = sr.json()
-    except Exception:
+    except httpx.HTTPError:
+        logger.warning("Services API unreachable for %s, skipping", coordinator_url)
         services_data = []
 
     # Build per-node, per-GPU running services map

tests/test_nodes.py

@@ -6,6 +6,7 @@ from pathlib import Path
 import pytest
 import yaml
 from fastapi.testclient import TestClient
+from unittest.mock import MagicMock, patch
 
 
 @pytest.fixture(autouse=True)
@@ -47,7 +48,6 @@ def test_list_nodes_returns_empty_when_no_coordinator(client):
     assert r.json() == []
 
 
-from unittest.mock import MagicMock, patch
 
 
 def _fake_nodes_response(nodes_json: list, services_json: list | None = None):