feat(llm): add MiniMax as a built-in provider

Two regional endpoints (global api.minimax.io, China api.minimaxi.com)
with separate API keys. Models M2.7 / M2.5 plus -highspeed variants,
204K context. Follows the existing provider-preset pattern.

#789 #609 #577 #546 #395 #378

tradingagents/llm_clients/factory.py

@@ -4,7 +4,9 @@ from .base_client import BaseLLMClient
 
 # Providers that use the OpenAI-compatible chat completions API
 _OPENAI_COMPATIBLE = (
-    "openai", "xai", "deepseek", "qwen", "glm", "ollama", "openrouter",
+    "openai", "xai", "deepseek", "qwen", "glm",
+    "minimax", "minimax-cn",
+    "ollama", "openrouter",
 )