fix(server): honour the legacy thresholds: config; repair the stale test suite

Since the built-in defaults were seeded into threshold_configs, the legacy
top-level thresholds: format was silently ignored. get_thresholds_for_host
now uses it whenever it was configured.

Tests: point imports at hbd.server/hbd.common, rewrite the DNS worker,
proto, datagram and threshold tests for the current API, add pytest-asyncio
to the dev extras and set testpaths so a bare pytest runs cleanly.

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

tests/test_threshold.py

@@ -19,7 +19,7 @@ from pathlib import Path
 # Add parent directory to path for imports
 sys.path.insert(0, str(Path(__file__).parent.parent))
 
-from hbd.threshold import (
+from hbd.server.threshold import (
     ThresholdChecker,
     ThresholdConfig,
     AlertLevel,
@@ -257,7 +257,16 @@ def test_threshold_checker_parsing():
     print(f"  ✓ Parsed {len(checker.thresholds)} thresholds correctly")
 
 
-def test_check_plugin_data():
+def _checker_with_notifications(config, notifications, monkeypatch):
+    """ThresholdChecker with no grace period that records notifications instead of sending them."""
+    monkeypatch.setattr(
+        ThresholdChecker, "_send_notification",
+        lambda self, host, lvl, message, *a: notifications.append(f"[{lvl}] {host} {message}"),
+    )
+    return ThresholdChecker({"grace": 0, **config})
+
+
+def test_check_plugin_data(monkeypatch):
     """Test checking plugin data against thresholds."""
     print("\nTest 6: Plugin data checking...")
     
@@ -277,11 +286,7 @@ def test_check_plugin_data():
     }
     
     notifications = []
-    
-    def notification_callback(msg):
-        notifications.append(msg)
-    
-    checker = ThresholdChecker(config, notification_callback=notification_callback)
+    checker = _checker_with_notifications(config, notifications, monkeypatch)
     alert_states = {}
     
     # First check - OK
@@ -353,12 +358,12 @@ def test_check_plugin_data():
     
     assert len(state_changes) == 2, "Two metrics should recover"
     assert len(notifications) == 2, "Two recovery notifications"
-    assert any("RECOVERED" in n for n in notifications), "Should have recovery notification"
+    assert any("RECOVER" in n for n in notifications), "Should have recovery notification"
     
     print("  ✓ Plugin data checking and notifications work")
 
 
-def test_nested_metrics():
+def test_nested_metrics(monkeypatch):
     """Test checking nested metrics like disk partitions."""
     print("\nTest 7: Nested metrics (partitions)...")
     
@@ -384,7 +389,7 @@ def test_nested_metrics():
     }
     
     notifications = []
-    checker = ThresholdChecker(config, notification_callback=lambda m: notifications.append(m))
+    checker = _checker_with_notifications(config, notifications, monkeypatch)
     alert_states = {}
     
     plugin_data = {