Return LogOutput object and update callers

Introduce a LogOutput dataclass (items, start, end) and change Log.output to return a LogOutput instead of a bare list. Update callers to unwrap items: api_log_get now reads log_output.items, and state_snapshot builds logs and log_end from the LogOutput and uses log_end for log_version. Also re-enable the @extensible decorator on AgentContext.output. These changes expose log metadata (start/end) alongside items for consumers.

python/api/api_log_get.py

@@ -44,7 +44,8 @@ class ApiLogGet(ApiHandler):
             start_pos = max(0, total_items - length)
 
             # Get log items from the calculated start position
-            log_items = context.log.output(start=start_pos)
+            log_output = context.log.output(start=start_pos)
+            log_items = log_output.items
 
             # Return log data with metadata
             return {