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>

pyproject.toml

@@ -58,6 +58,7 @@ all = [
 # Development dependencies
 dev = [
   "pytest>=7.0",
+  "pytest-asyncio>=0.23",
   "pytest-cov>=4.0",
   "flake8>=5.0",
   "mypy>=1.10",
@@ -90,3 +91,6 @@ max-line-length = 111
 
 [tool.pylint.format]
 max-line-length = 111
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]