fix: update test_stats_handler.py for langchain_core >=1.0 compatibility

In langchain_core >=1.0 plain Generation no longer stores a .message
attribute - that only exists on ChatGeneration. Tests were constructing
Generation(message=AIMessage(...)) which silently dropped the message,
making hasattr(generation, "message") return False and skipping the
token-counting path (all usage assertions failed with 0).

- Replace Generation(message=...) with ChatGeneration(message=AIMessage(...))
  in test_stats_handler_on_llm_end_with_usage and thread_safety test
- Use UsageMetadata(input_tokens=N, output_tokens=N, total_tokens=N)
  instead of bare dict (total_tokens is required in langchain_core 1.2+)
- Pass usage_metadata via AIMessage constructor instead of post-init
  attribute assignment (avoids pydantic validation bypass)
- Keep Generation(text=...) in test_stats_handler_on_llm_end_no_usage
  (correctly tests the "no usage" branch — plain Generation has no .message)

Co-authored-by: aguzererler <6199053+aguzererler@users.noreply.github.com>
Agent-Logs-Url: https://github.com/aguzererler/TradingAgents/sessions/ce079791-08ef-4f2e-9f31-a1ae6a26b4cb

tests/cli/test_stats_handler.py

@@ -1,8 +1,9 @@
 import threading
 import pytest
 from cli.stats_handler import StatsCallbackHandler
-from langchain_core.outputs import LLMResult, Generation
+from langchain_core.outputs import LLMResult, Generation, ChatGeneration
 from langchain_core.messages import AIMessage
+from langchain_core.messages.ai import UsageMetadata
 
 def test_stats_handler_initial_state():
     handler = StatsCallbackHandler()
@@ -35,11 +36,10 @@ def test_stats_handler_on_tool_start():
 def test_stats_handler_on_llm_end_with_usage():
     handler = StatsCallbackHandler()
 
-    # Mock usage metadata
-    usage_metadata = {"input_tokens": 10, "output_tokens": 20}
-    message = AIMessage(content="test response")
-    message.usage_metadata = usage_metadata
-    generation = Generation(message=message, text="test response")
+    # ChatGeneration wraps chat messages; Generation (plain text) has no .message attr.
+    usage_metadata = UsageMetadata(input_tokens=10, output_tokens=20, total_tokens=30)
+    message = AIMessage(content="test response", usage_metadata=usage_metadata)
+    generation = ChatGeneration(message=message)
     response = LLMResult(generations=[[generation]])
 
     handler.on_llm_end(response)
@@ -83,11 +83,10 @@ def test_stats_handler_thread_safety():
             handler.on_llm_start({}, [])
             handler.on_tool_start({}, "")
 
-            # Mock usage metadata for on_llm_end
-            usage_metadata = {"input_tokens": 1, "output_tokens": 1}
-            message = AIMessage(content="x")
-            message.usage_metadata = usage_metadata
-            generation = Generation(message=message, text="x")
+            # ChatGeneration wraps chat messages with usage_metadata
+            usage_metadata = UsageMetadata(input_tokens=1, output_tokens=1, total_tokens=2)
+            message = AIMessage(content="x", usage_metadata=usage_metadata)
+            generation = ChatGeneration(message=message)
             response = LLMResult(generations=[[generation]])
             handler.on_llm_end(response)