fix(signal): raise on signal-cli error response so send is retriable

_send_http_request collapses every exception path into a {"error": ...}
dict, so the if "error" in response branch inside send() is the only
place where send failures surface. Logging-only there meant the
ChannelManager retry mechanism never fired. Raise RuntimeError so the
base-class retry path is exercised; the outer try/except already
re-raises into the caller.

Addresses review comment on PR #3852.

nanobot/channels/signal.py

@@ -506,6 +506,7 @@ class SignalChannel(BaseChannel):
 
                 if "error" in response:
                     self.logger.error(f"Error sending Signal message: {response['error']}")
+                    raise RuntimeError(f"signal-cli send failed: {response['error']}")
                 else:
                     self.logger.debug(
                         f"Signal message sent, timestamp: {response.get('result', {}).get('timestamp')}"