refactor(llm_clients): 重构工厂模式实现，添加动态导入和依赖检查

将硬编码的导入改为动态导入，并添加对缺失依赖的检查
为codex_oauth添加新的客户端支持
更新provider参数文档说明

tradingagents/llm_clients/factory.py

@@ -1,9 +1,6 @@
 from typing import Optional
 
 from .base_client import BaseLLMClient
-from .openai_client import OpenAIClient
-from .anthropic_client import AnthropicClient
-from .google_client import GoogleClient
 
 
 def create_llm_client(
@@ -15,7 +12,7 @@ def create_llm_client(
     """Create an LLM client for the specified provider.
 
     Args:
-        provider: LLM provider (openai, anthropic, google, xai, ollama, openrouter)
+        provider: LLM provider (openai, codex_oauth, anthropic, google, xai, ollama, openrouter)
         model: Model name/identifier
         base_url: Optional base URL for API endpoint
         **kwargs: Additional provider-specific arguments
@@ -29,15 +26,57 @@ def create_llm_client(
     provider_lower = provider.lower()
 
     if provider_lower in ("openai", "ollama", "openrouter"):
+        try:
+            from .openai_client import OpenAIClient
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                "Missing dependency for OpenAI-compatible providers. "
+                "Install `langchain-openai`."
+            ) from exc
+
         return OpenAIClient(model, base_url, provider=provider_lower, **kwargs)
 
+    if provider_lower == "codex_oauth":
+        try:
+            from .codex_oauth_client import CodexOAuthClient
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                "Missing dependency for codex_oauth provider. "
+                "Install `langchain-codex-oauth`."
+            ) from exc
+
+        return CodexOAuthClient(model, base_url, **kwargs)
+
     if provider_lower == "xai":
+        try:
+            from .openai_client import OpenAIClient
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                "Missing dependency for xAI provider. Install `langchain-openai`."
+            ) from exc
+
         return OpenAIClient(model, base_url, provider="xai", **kwargs)
 
     if provider_lower == "anthropic":
+        try:
+            from .anthropic_client import AnthropicClient
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                "Missing dependency for Anthropic provider. "
+                "Install `langchain-anthropic`."
+            ) from exc
+
         return AnthropicClient(model, base_url, **kwargs)
 
     if provider_lower == "google":
+        try:
+            from .google_client import GoogleClient
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                "Missing dependency for Google provider. "
+                "Install `langchain-google-genai`."
+            ) from exc
+
         return GoogleClient(model, base_url, **kwargs)
 
     raise ValueError(f"Unsupported LLM provider: {provider}")