feat(llm): add NVIDIA NIM, Kimi, Groq, and Mistral providers

Each is a one-row entry in the OpenAI-compatible provider registry (base_url,
key env, CLI option); the model is user-specified since they serve many models.

tradingagents/llm_clients/validators.py

@@ -3,9 +3,13 @@
 from .model_catalog import get_known_models
 
 
-# Providers whose model names are user-defined (local servers, relays, any
-# OpenAI-compatible endpoint), so any model string is accepted without warning.
-_ANY_MODEL_PROVIDERS = ("ollama", "openrouter", "openai_compatible")
+# Providers whose model names are user-defined (local servers, relays, hosted
+# OpenAI-compatible endpoints serving many models), so any model string is
+# accepted without warning.
+_ANY_MODEL_PROVIDERS = (
+    "ollama", "openrouter", "openai_compatible",
+    "mistral", "kimi", "groq", "nvidia",
+)
 
 VALID_MODELS = {
     provider: models