fix: correct zero-safe pathconf checks and connectivity prefix match

- Use `is not None` for pathconf values so 0 is not silently dropped
- Broaden connectivity prefix check to catch bare "connectivity" key

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/threshold.py

@@ -1556,7 +1556,7 @@ class ThresholdChecker:
             for mp in host.alert_states:
                 # connectivity.* and rtt are managed by the connection state
                 # machine, not by threshold config — never purge them.
-                if mp == "rtt" or mp.startswith("connectivity."):
+                if mp == "rtt" or mp.startswith("connectivity"):
                     continue
                 if self._find_threshold(configured, mp)[0] is not None:
                     continue