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_udp.py

@@ -1,5 +1,5 @@
-from hbd.udp import parse_message
-from hbd.proto import dicttos
+from hbd.server.udp import parse_message
+from hbd.common.proto import dicttos
 
 
 def test_parse_message_uncompressed():