feat(llm): unify OpenAI-compatible providers behind a registry + generic endpoint

The OpenAI-compatible family (openai, xAI, DeepSeek, Qwen, GLM, MiniMax,
OpenRouter, Ollama) all speak the same Chat Completions API and differ only by
base_url, key, and two narrow wire-format quirks already isolated in subclasses.
Replace the scattered base-URL dict, key handling, and client-class branches with
one ProviderSpec registry that get_llm and the factory drive off; provider quirks
stay in their subclasses. Add a generic "openai_compatible" provider for any
OpenAI-compatible server (vLLM, LM Studio, llama.cpp, relays) via backend_url +
optional key — adding a provider is now one registry row. Native Anthropic/Google
keep their own clients (genuinely different APIs). Also fixes the env backend URL
being ignored when the provider was chosen interactively (#978).

tradingagents/llm_clients/validators.py

@@ -3,21 +3,25 @@
 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")
+
 VALID_MODELS = {
     provider: models
     for provider, models in get_known_models().items()
-    if provider not in ("ollama", "openrouter")
+    if provider not in _ANY_MODEL_PROVIDERS
 }
 
 
 def validate_model(provider: str, model: str) -> bool:
     """Check if model name is valid for the given provider.
 
-    For ollama, openrouter - any model is accepted.
+    For ollama, openrouter, and openai_compatible - any model is accepted.
     """
     provider_lower = provider.lower()
 
-    if provider_lower in ("ollama", "openrouter"):
+    if provider_lower in _ANY_MODEL_PROVIDERS:
         return True
 
     if provider_lower not in VALID_MODELS: