feat(server): Add OpenAI /v1/models/{model_id} endpoint

[phillipmunn]

In Microsoft Visual Studio, you can use local AI endpoints. When you point it at the OpenAI compatible API, it makes a request to:

GET http://127.0.0.1:1919/v1/models

and returns:

{
  "object": "list",
  "data": [
    {
      "id": "Qwen3.6-35B-A3B-NVFP4",
      ...
    }
  ]
}

Afterwards, Visual Studio then makes a follow up request to:

GET http://127.0.0.1:1919/v1/models/Qwen3.6-35B-A3B-NVFP4 and currently gets a 404, even though the model really is loaded. This then prevents FreeToken server from being used as Visual Studio assumes the OpenAPI server and/or model name is invalid.

This is a small OpenAI-compatibility gap. The OpenAI API has a “retrieve model” operation corresponding to GET /v1/models/{model}, and some clients evidently use it to validate a manually specified model name.

This change leaves the existing /v1/models endpoint unchanged and only adds the missing OpenAI-compatible retrieve-model operation. The existing route currently constructs its ModelCard using the same context-length and reasoning-effort metadata shown above.

[gdevenyi]

If we're going to expand the OpenAI endpoints, maybe we should audit for any other missing features too