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/__init__.py

@@ -0,0 +1,4 @@
+from .base_client import BaseLLMClient
+from .factory import create_llm_client
+
+__all__ = ["BaseLLMClient", "create_llm_client"]