agent: Allow customizing temperature by provider/model (#30033)

Adds a new `agent.model_parameters` setting that allows the user to
specify a custom temperature for a provider AND/OR model:

```json5
    "model_parameters": [
      // To set parameters for all requests to OpenAI models:
      {
        "provider": "openai",
        "temperature": 0.5
      },
      // To set parameters for all requests in general:
      {
        "temperature": 0
      },
      // To set parameters for a specific provider and model:
      {
        "provider": "zed.dev",
        "model": "claude-3-7-sonnet-latest",
        "temperature": 1.0
      }
    ],
```

Release Notes:

- agent: Allow customizing temperature by provider/model

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

docs/src/configuring-zed.md

@@ -3070,14 +3070,14 @@ Run the `theme selector: toggle` action in the command palette to see a current
 }
 ```
 
-## Assistant Panel
+## Agent
 
-- Description: Customize assistant panel
-- Setting: `assistant`
+- Description: Customize agent behavior
+- Setting: `agent`
 - Default:
 
 ```json
-"assistant": {
+"agent": {
   "version": "2",
   "enabled": true,
   "button": true,