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/factory.py

@@ -4,7 +4,6 @@ from .base_client import BaseLLMClient
 from .openai_client import OpenAIClient
 from .anthropic_client import AnthropicClient
 from .google_client import GoogleClient
-from .vllm_client import VLLMClient
 
 
 def create_llm_client(
@@ -16,7 +15,7 @@ def create_llm_client(
     """Create an LLM client for the specified provider.
 
     Args:
-        provider: LLM provider (openai, anthropic, google, xai, ollama, openrouter, vllm)
+        provider: LLM provider (openai, anthropic, google, xai, ollama, openrouter)
         model: Model name/identifier
         base_url: Optional base URL for API endpoint
         **kwargs: Additional provider-specific arguments
@@ -41,7 +40,4 @@ def create_llm_client(
     if provider_lower == "google":
         return GoogleClient(model, base_url, **kwargs)
 
-    if provider_lower == "vllm":
-        return VLLMClient(model, base_url, **kwargs)
-
     raise ValueError(f"Unsupported LLM provider: {provider}")