feat(opencode): dynamic model discovery for local providers (LM Studio, llama.cpp, etc.)

Issue for this PR

Closes #6231

Type of change

• [ ] Bug fix
• [x] New feature
• [ ] Refactor / code improvement
• [ ] Documentation

What does this PR do?

Adds an option for dynamic model list population for OpenAI-compatible providers supporting /model API (as opposed to manually typing them in opencode.json):

• There's a new option which can be added to a "provider" in opencode.json: "dynamicModelList": true
• If the option is not set, the old behavior takes precedence (backward compatibility)
• When set to true issues an API request and retrieves the list of the models supported by the provider
• If the list is returned successfully, populates the models from the response
• In the explicit list of models is provided, it takes precedence even if the "dynamicModelList": true flag is set

This change makes working with local AI inference engines much nicer, because it eliminates the need to update opencode.json every time a new model is added, or if the engine is started with the specific model as a parameter. The model list is now completely driven by the inference engine itself.

Example configuration (opencode.json):

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "llama.cpp": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "llama.cpp (local)",
      "options": {
        "baseURL": "http://127.0.0.1:8080/v1"
      },
      "dynamicModelList": true
    },
    "lm_studio": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "LM Studio (local)",
      "options": {
        "baseURL": "http://127.0.0.1:1234/v1",
      },
      "dynamicModelList": true
    }
  }
}

will result in the following model list in OpenCode, for LM Studio with "Just-in-Time Model Loading" enabled:

<img width="340" height="61" alt="image" src="https://github.com/user-attachments/assets/6cb49d59-3510-4986-bb8d-73638290dee3" />

or llama.cpp started without specifying a model:

<img width="340" height="63" alt="image" src="https://github.com/user-attachments/assets/ccd7b181-a307-4b0d-a47f-efa1314a48b8" />

The selected model will be loaded by the inference engine once the first /chat/completion request is received.

If, however, the inference engines only serving specific models, only those models will be returned and available in OpenCode model selector. For LM Studio with "Just-in-Time Model Loading" disabled:

<img width="300" height="58" alt="image" src="https://github.com/user-attachments/assets/781400f7-7ba6-4319-93a7-768358cfaa1e" />

or llama.cpp started with a specific model:

<img width="300" height="58" alt="image" src="https://github.com/user-attachments/assets/5bd14412-7461-4cb2-adaa-1616bf96c875" />

I'm aware that there's a number of opened PRs on this issue already (i.e. #15732, #13234), however I believe this is the most robust implementation because:

• it doesn't hard-code any specific provider name
• it maintains the existing behavior by default (if the new flag is not used)
• it works with any provider which supports /model endpoint (not even just a local one, i.e. Ollama Cloud also works just fine)
• adds a bunch of tests

🤖 Developed with some help from OpenCode running against a local model hosted in llama.cpp!

How did you verify your code works?

Run OpenCode with models hosted in LM Studio and llama.cpp. Tried various scenarios:

• with a single model loaded vs dynamic model loading (see the screenshots above)
• with/without auth
• with the inference engine down

Also tested it agains Ollama Cloud

Screenshots / recordings

See above

Checklist

• [x] I have tested my changes locally
• [x] I have not included unrelated changes in this PR