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>

hbd/server/threshold.py

@@ -827,7 +827,7 @@ class ThresholdChecker:
             Dictionary of thresholds for this host
         """
         # Legacy mode: single threshold set for all hosts
-        if self.thresholds and not self.threshold_configs:
+        if self.thresholds:
             return self.thresholds
 
         if not self.threshold_configs: