Include OpenRouter support within API Keys supported services

Following up because the thread Cursor staff asked us to consolidate into (#161060) auto-closed on 27th June, so there is currently no open place to add product feedback on native OpenRouter BYOK.

Use case: Institutional OpenRouter key (employer-provided), Cursor Ultra quota exhausted mid-month. Need large model access without LiteLLM/VPS workarounds.

Ideal, native, UX

  1. Dedicated OpenRouter provider in Settings → API Keys (not hijacking the OpenAI override slot)
  2. Model catalog sync or “import from OpenRouter” so models appear in the chat picker
  3. Coexistence: subscription models (Composer, included Claude) and OpenRouter models in the same dropdown — per-model routing, not a global base-URL override
  4. Toggle: enable/disable OpenRouter without breaking built-ins or requiring sign-out
  5. Agent mode should route to OpenRouter when an OpenRouter model is selected