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/model_catalog.py

@@ -175,6 +175,12 @@ MODEL_OPTIONS: ProviderModeOptions = {
             ("Custom model ID", "custom"),
         ],
     },
+    # Generic OpenAI-compatible endpoint: the model is whatever the user's
+    # server serves, so only "Custom model ID" is offered.
+    "openai_compatible": {
+        "quick": [("Custom model ID", "custom")],
+        "deep": [("Custom model ID", "custom")],
+    },
 }