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/validators.py

@@ -0,0 +1,69 @@
+from typing import Dict, List
+
+VALID_MODELS: Dict[str, List[str]] = {
+    "openai": [
+        "gpt-4o",
+        "gpt-4o-mini",
+        "gpt-4-turbo",
+        "gpt-4",
+        "gpt-3.5-turbo",
+        "o1",
+        "o1-mini",
+        "o1-preview",
+        "o3-mini",
+        "gpt-5-nano",
+        "gpt-5-mini",
+        "gpt-5",
+    ],
+    "anthropic": [
+        "claude-3-5-sonnet-20241022",
+        "claude-3-5-haiku-20241022",
+        "claude-3-opus-20240229",
+        "claude-3-sonnet-20240229",
+        "claude-3-haiku-20240307",
+        "claude-sonnet-4-20250514",
+        "claude-haiku-4-5-20251001",
+        "claude-opus-4-5-20251101",
+    ],
+    "google": [
+        "gemini-1.5-pro",
+        "gemini-1.5-flash",
+        "gemini-2.0-flash",
+        "gemini-2.0-flash-lite",
+        "gemini-2.5-pro-preview-05-06",
+        "gemini-2.5-flash-preview-05-20",
+        "gemini-3-pro-preview",
+        "gemini-3-flash-preview",
+    ],
+    "xai": [
+        "grok-beta",
+        "grok-2",
+        "grok-2-mini",
+        "grok-3",
+        "grok-3-mini",
+    ],
+    "ollama": [],
+    "openrouter": [],
+    "vllm": [],
+}
+
+
+def validate_model(provider: str, model: str) -> bool:
+    """Validate that a model is supported by the provider.
+
+    For ollama, openrouter, and vllm, any model is accepted.
+    For other providers, checks against VALID_MODELS.
+    """
+    provider_lower = provider.lower()
+
+    if provider_lower in ("ollama", "openrouter", "vllm"):
+        return True
+
+    if provider_lower not in VALID_MODELS:
+        return False
+
+    valid = VALID_MODELS[provider_lower]
+    if not valid:
+        return True
+
+    return model in valid