feat: add multi-provider LLM support with factory pattern

- Add tradingagents/llm_clients/ with unified factory pattern
- Support OpenAI, Anthropic, Google, xAI, OpenRouter, Ollama, vLLM
- Replace direct LLM imports in trading_graph.py with create_llm_client()
- Handle provider-specific params (reasoning_effort, thinking_config)

tradingagents/llm_clients/base_client.py

@@ -0,0 +1,21 @@
+from abc import ABC, abstractmethod
+from typing import Any, Optional
+
+
+class BaseLLMClient(ABC):
+    """Abstract base class for LLM clients."""
+
+    def __init__(self, model: str, base_url: Optional[str] = None, **kwargs):
+        self.model = model
+        self.base_url = base_url
+        self.kwargs = kwargs
+
+    @abstractmethod
+    def get_llm(self) -> Any:
+        """Return the configured LLM instance."""
+        pass
+
+    @abstractmethod
+    def validate_model(self) -> bool:
+        """Validate that the model is supported by this client."""
+        pass