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/api_key_env.py

@@ -32,6 +32,10 @@ PROVIDER_API_KEY_ENV: dict[str, Optional[str]] = {
     "openrouter": "OPENROUTER_API_KEY",
     # Local runtimes do not authenticate.
     "ollama":     None,
+    # Generic OpenAI-compatible endpoint: the client reads this when set (keyed
+    # relays), but it is marked key-optional in the provider registry so the CLI
+    # never forces a prompt and keyless local servers still work.
+    "openai_compatible": "OPENAI_COMPATIBLE_API_KEY",
 }