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/api_key_env.py

@@ -30,6 +30,12 @@ PROVIDER_API_KEY_ENV: dict[str, Optional[str]] = {
     "minimax":    "MINIMAX_API_KEY",
     "minimax-cn": "MINIMAX_CN_API_KEY",
     "openrouter": "OPENROUTER_API_KEY",
+    # Additional hosted OpenAI-compatible providers (model is user-specified).
+    # kimi -> Moonshot AI; nvidia -> NVIDIA NIM.
+    "mistral":    "MISTRAL_API_KEY",
+    "kimi":       "MOONSHOT_API_KEY",
+    "groq":       "GROQ_API_KEY",
+    "nvidia":     "NVIDIA_API_KEY",
     # Local runtimes do not authenticate.
     "ollama":     None,
     # Generic OpenAI-compatible endpoint: the client reads this when set (keyed