test(host_agent): update existing collector tests for new agent API

collect_cpu now returns (aggregate, [per_core]) and build_sample takes a
rate-state dict, so the pre-existing tests that asserted the old float
return / no-state signature needed updating. Mocks the new collectors
(temps/psi/net/diskio) for determinism.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/plugins/host_agent/test_agent_collectors.py

@@ -56,10 +56,11 @@ def test_collect_cpu_reads_twice():
 
     with patch.object(a, "_read_file", _read), \
          patch.object(a.time, "sleep", lambda _s: None):
-        pct = a.collect_cpu(sample_window=0.0)
+        pct, cores = a.collect_cpu(sample_window=0.0)
     # totals: 1000 → 2000 (delta 1000). idle+iowait: 850 → 1700 (delta 850).
-    # busy = 150. pct = 15.0.
+    # busy = 150. pct = 15.0. No cpuN lines in the fixture → no per-core values.
     assert pct == pytest.approx(15.0, abs=0.1)
+    assert cores == []
 
 
 def test_collect_storage():