fix(claude-agent): raise error instead of silently returning empty messages

Returning 0 messages means the model was unreachable (VPN down, proxy
error, etc). Logging a warning and succeeding caused Temporal to mark
the activity as completed, the workflow to continue pointlessly, and
the root cause to be invisible.

Raising RuntimeError lets Temporal retry the activity or surface the
failure clearly in the workflow timeline.

Author: gabrycina

src/agentex/lib/core/temporal/plugins/claude_agents/activities.py

@@ -146,9 +146,10 @@ async def run_claude_agent_activity(
 
         results = handler.get_results()
         if not results.get("messages"):
-            logger.warning(
+            raise RuntimeError(
                 f"[run_claude_agent_activity] Claude returned 0 messages — "
-                f"possible model connectivity issue (task_id={task_id}, workspace={workspace_path})"
+                f"possible model connectivity issue (task_id={task_id}, workspace={workspace_path}). "
+                f"Check that the model API is reachable."
             )
         logger.debug(f"Returning results with keys: {results.keys()}")
         return results