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/TODO.md

@@ -0,0 +1,24 @@
+# LLM Clients - Consistency Improvements
+
+## Issues to Fix
+
+### 1. `validate_model()` is never called
+- Add validation call in `get_llm()` with warning (not error) for unknown models
+
+### 2. Inconsistent parameter handling
+| Client | API Key Param | Special Params |
+|--------|---------------|----------------|
+| OpenAI | `api_key` | `reasoning_effort` |
+| Anthropic | `api_key` | `thinking_config` → `thinking` |
+| Google | `google_api_key` | `thinking_budget` |
+
+**Fix:** Standardize with unified `api_key` that maps to provider-specific keys
+
+### 3. `base_url` accepted but ignored
+- `AnthropicClient`: accepts `base_url` but never uses it
+- `GoogleClient`: accepts `base_url` but never uses it (correct - Google doesn't support it)
+
+**Fix:** Remove unused `base_url` from clients that don't support it
+
+### 4. Update validators.py with models from CLI
+- Sync `VALID_MODELS` dict with CLI model options after Feature 2 is complete