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_nagios_runner.py

@@ -1,15 +1,11 @@
 import asyncio
 import logging
-import os
 import stat
 
-from hbd.client.plugins.nagios_runner import (
-    NagiosRunnerPlugin,
-    NAGIOS_OK,
-    NAGIOS_WARNING,
-    NAGIOS_CRITICAL,
-    NAGIOS_UNKNOWN,
-)
+from hbd.client.plugins.nagios_runner import NagiosRunnerPlugin, NAGIOS_UNKNOWN
+
+# Nagios plugin exit codes (the module itself only needs UNKNOWN)
+NAGIOS_OK, NAGIOS_WARNING, NAGIOS_CRITICAL = 0, 1, 2
 
 
 def test_no_commands_sets_skip_reason():