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/google_client.py

@@ -38,7 +38,7 @@ class GoogleClient(BaseLLMClient):
         """Return configured ChatGoogleGenerativeAI instance."""
         llm_kwargs = {"model": self.model}
 
-        for key in ("timeout", "max_retries", "google_api_key"):
+        for key in ("timeout", "max_retries", "google_api_key", "callbacks"):
             if key in self.kwargs:
                 llm_kwargs[key] = self.kwargs[key]