Feature request: add a simple playground to test models directly in the UI

Title: Feature request: add a simple playground to test models directly in the UI It would be very helpful to have a small playground in OmniRoute so users can quickly test whether a model actually works, instead of only seeing it listed in `/v1/models`. Suggested features:

• pick a provider
• pick a model
• choose endpoint type (`chat`, `responses`, `images`, maybe `embeddings`)
• enter a simple test payload/prompt
• run the request directly from the UI
• show raw request/response and upstream