fix: stop synthetic rtt_* keys from masking real plugin-data checks

host.plugin_data now always holds rtt_ipv4/rtt_ipv6 history after the
first heartbeat, which broke two pre-existing behaviors that assumed
plugin_data reflects only real client-collected data:

- The request_update ACK gate in handle_datagram() never fired again
  after a host's first heartbeat, so stale OS/agent-version info was
  never refreshed after a reconnect (only a client restart fixed it).
- plugin_data.clear() on every UP transition wiped rtt_* history too,
  so a flaky host could never accumulate a useful RTT graph.

Add _is_rtt_key()/_has_real_plugin_data() helpers so both spots treat
rtt_* keys as synthetic: the gate now looks only at real plugin data,
and the recovery clear only drops real plugin keys, preserving RTT
history across reconnects.

Also fixes two pre-existing E127 continuation-indent flake8 issues in
tests/test_udp_rtt_history.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEimzMv4Q5EjFg3hoiZ69T

hbd/server/udp.py

@@ -319,6 +319,16 @@ def restore_connection_timers(hbdclass, ctx):
     logger.info("Restored timers for %d connection(s)", restored)
 
 
+def _is_rtt_key(plugin_name: str) -> bool:
+    """True for synthetic RTT-history keys (rtt_ipv4/rtt_ipv6), not real plugin data."""
+    return plugin_name.startswith("rtt_")
+
+
+def _has_real_plugin_data(plugin_data: dict) -> bool:
+    """True if plugin_data holds any real (non-RTT) client-collected plugin data."""
+    return any(not _is_rtt_key(k) for k in plugin_data)
+
+
 def handle_datagram(msg: dict, addr, transport, ctx: dict):
     """Handle a parsed datagram message.
 
@@ -385,7 +395,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         host.doesack = msg.get("acks", -1)
         # send ACK back; ask client to resend plugin info when we have none yet
         rmsg = {"time": time.time()}
-        if not host.plugin_data:
+        if not _has_real_plugin_data(host.plugin_data):
             rmsg["request_update"] = 1
         opkt = dicttos("ACK", rmsg)
         try:
@@ -516,7 +526,8 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         # cleanly from the first two post-boot samples.
         for pname in list(host.plugin_timers):
             host.cancel_plugin_timer(pname)
-        host.plugin_data.clear()
+        for pname in [k for k in host.plugin_data if not _is_rtt_key(k)]:
+            del host.plugin_data[pname]
         stale_plugin_keys = [
             k for k in host.alert_states
             if k not in ("rtt",) and not k.startswith("connectivity.")