Fix LM Studio model detection (#3800) — the settings dropdown was silently dropping models for every auto-discovered provider except Ollama / OpenAI / Anthropic / OpenAI-Compatible-Endpoint, including LM Studio, Llama.cpp, Google Gemini, xAI, OpenRouter, and IONOS. While investigating, also fixed a separate credential-leak class where the route's auto-discovery loop sent every other provider's configured API key in the `Authorization` header to the local LM Studio / llama-server endpoint on every settings-page load.
