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/validators.py

@@ -69,11 +69,11 @@ VALID_MODELS = {
 def validate_model(provider: str, model: str) -> bool:
     """Check if model name is valid for the given provider.
 
-    For ollama, openrouter, vllm - any model is accepted.
+    For ollama, openrouter - any model is accepted.
     """
     provider_lower = provider.lower()
 
-    if provider_lower in ("ollama", "openrouter", "vllm"):
+    if provider_lower in ("ollama", "openrouter"):
         return True
 
     if provider_lower not in VALID_MODELS: