feat: add footer statistics tracking with LangChain callbacks

- Add StatsCallbackHandler for tracking LLM calls, tool calls, and tokens
- Integrate callbacks into TradingAgentsGraph and all LLM clients
- Dynamic agent/report counts based on selected analysts
- Fix report completion counting (tied to agent completion)

tradingagents/llm_clients/openai_client.py

@@ -60,7 +60,7 @@ class OpenAIClient(BaseLLMClient):
         elif self.base_url:
             llm_kwargs["base_url"] = self.base_url
 
-        for key in ("timeout", "max_retries", "reasoning_effort", "api_key"):
+        for key in ("timeout", "max_retries", "reasoning_effort", "api_key", "callbacks"):
             if key in self.kwargs:
                 llm_kwargs[key] = self.kwargs[key]