fix: add explicit UTF-8 encoding to all file open() calls

Prevents UnicodeEncodeError on Windows where the default encoding
(cp1252/gbk) cannot handle Unicode characters in LLM output.

Closes #77, closes #114, closes #126, closes #215, closes #332

tradingagents/graph/trading_graph.py

@@ -260,6 +260,7 @@ class TradingAgentsGraph:
         with open(
             f"eval_results/{self.ticker}/TradingAgentsStrategy_logs/full_states_log_{trade_date}.json",
             "w",
+            encoding="utf-8",
         ) as f:
             json.dump(self.log_states_dict, f, indent=4)