fix(signal): track SSE keepalive comments as connection activity

signal-cli sends SSE comment lines (':') as keepalives every ~15s. The
SSE listener only counted 'data:' lines as activity, so the health
monitor reported false idle warnings every 2 minutes during quiet
periods. Recognize ':' lines as valid activity per the SSE spec.

Salvaged from PR #2938 by ticketclosed-wontfix.

gateway/platforms/signal.py

@@ -279,6 +279,12 @@ class SignalAdapter(BasePlatformAdapter):
                             line = line.strip()
                             if not line:
                                 continue
+                            # SSE keepalive comments (":") prove the connection
+                            # is alive — update activity so the health monitor
+                            # doesn't report false idle warnings.
+                            if line.startswith(":"):
+                                self._last_sse_activity = time.time()
+                                continue
                             # Parse SSE data lines
                             if line.startswith("data:"):
                                 data_str = line[5:].strip()