fix: fix issue with linking tools and adding new tools (#1988)

Co-authored-by: Matt Zhou <mattzh1314@gmail.com>

letta/agent.py

@@ -3,7 +3,6 @@ import inspect
 import traceback
 import warnings
 from abc import ABC, abstractmethod
-from lib2to3.fixer_util import is_list
 from typing import List, Literal, Optional, Tuple, Union
 
 from tqdm import tqdm
@@ -252,7 +251,7 @@ class Agent(BaseAgent):
             warnings.warn("Tool rules only work reliably for the latest OpenAI models that support structured outputs.")
         # add default rule for having send_message be a terminal tool
 
-        if not is_list(agent_state.tool_rules):
+        if agent_state.tool_rules is None:
             agent_state.tool_rules = []
         agent_state.tool_rules.append(TerminalToolRule(tool_name="send_message"))
         self.tool_rules_solver = ToolRulesSolver(tool_rules=agent_state.tool_rules)