Add capabilities to OpenAI-compatible model settings (#36370)

### TL;DR
* Adds `capabilities` configuration for OpenAI-compatible models
* Relates to
https://github.com/zed-industries/zed/issues/36215#issuecomment-3193920491

### Summary
This PR introduces support for configuring model capabilities for
OpenAI-compatible language models. The implementation addresses the
issue that not all OpenAI-compatible APIs support the same features -
for example, Cerebras' API explicitly does not support
`parallel_tool_calls` as documented in their [OpenAI compatibility
guide](https://inference-docs.cerebras.ai/resources/openai#currently-unsupported-openai-features).

### Changes

1. **Model Capabilities Structure**:
- Added `ModelCapabilityToggles` struct for UI representation with
boolean toggle states
- Implemented proper parsing of capability toggles into
`ModelCapabilities`

2. **UI Updates**:
- Modified the "Add LLM Provider" modal to include checkboxes for each
capability
- Each OpenAI-compatible model can now be configured with its specific
capabilities through the UI

3. **Configuration File Structure**:
- Updated the settings schema to support a `capabilities` object for
each `openai_compatible` model
- Each capability (`tools`, `images`, `parallel_tool_calls`,
`prompt_cache_key`) can be individually specified per model

### Example Configuration

```json
{
  "openai_compatible": {
    "Cerebras": {
      "api_url": "https://api.cerebras.ai/v1",
      "available_models": [
        {
          "name": "gpt-oss-120b",
          "max_tokens": 131000,
          "capabilities": {
            "tools": true,
            "images": false,
            "parallel_tool_calls": false,
            "prompt_cache_key": false
          }
        }
      ]
    }
  }
}
```

### Tests Added

- Added tests to verify default capability values are correctly applied
- Added tests to verify that deselected toggles are properly parsed as
`false`
- Added tests to verify that mixed capability selections work correctly

Thanks to @osyvokon for the desired `capabilities` configuration
structure!


Release Notes:

- OpenAI-compatible models now have configurable capabilities (#36370;
thanks @calesennett)

---------

Co-authored-by: Oleksiy Syvokon <oleksiy@zed.dev>

docs/src/ai/llm-providers.md

@@ -427,7 +427,7 @@ Custom models will be listed in the model dropdown in the Agent Panel.
 Zed supports using [OpenAI compatible APIs](https://platform.openai.com/docs/api-reference/chat) by specifying a custom `api_url` and `available_models` for the OpenAI provider.
 This is useful for connecting to other hosted services (like Together AI, Anyscale, etc.) or local models.
 
-You can add a custom, OpenAI-compatible model via either via the UI or by editing your `settings.json`.
+You can add a custom, OpenAI-compatible model either via the UI or by editing your `settings.json`.
 
 To do it via the UI, go to the Agent Panel settings (`agent: open settings`) and look for the "Add Provider" button to the right of the "LLM Providers" section title.
 Then, fill up the input fields available in the modal.
@@ -443,7 +443,13 @@ To do it via your `settings.json`, add the following snippet under `language_mod
         {
           "name": "mistralai/Mixtral-8x7B-Instruct-v0.1",
           "display_name": "Together Mixtral 8x7B",
-          "max_tokens": 32768
+          "max_tokens": 32768,
+          "capabilities": {
+            "tools": true,
+            "images": false,
+            "parallel_tool_calls": false,
+            "prompt_cache_key": false
+          }
         }
       ]
     }
@@ -451,6 +457,13 @@ To do it via your `settings.json`, add the following snippet under `language_mod
 }
 ```
 
+By default, OpenAI-compatible models inherit the following capabilities:
+
+- `tools`: true (supports tool/function calling)
+- `images`: false (does not support image inputs)
+- `parallel_tool_calls`: false (does not support `parallel_tool_calls` parameter)
+- `prompt_cache_key`: false (does not support `prompt_cache_key` parameter)
+
 Note that LLM API keys aren't stored in your settings file.
 So, ensure you have it set in your environment variables (`OPENAI_API_KEY=<your api key>`) so your settings can pick it up.