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>

crates/git_ui/src/git_panel.rs

@@ -1735,6 +1735,8 @@ impl GitPanel {
             }
         });
 
+        let temperature = AssistantSettings::temperature_for_model(&model, cx);
+
         self.generate_commit_message_task = Some(cx.spawn(async move |this, cx| {
              async move {
                 let _defer = cx.on_drop(&this, |this, _cx| {
@@ -1773,7 +1775,7 @@ impl GitPanel {
                     }],
                     tools: Vec::new(),
                     stop: Vec::new(),
-                    temperature: None,
+                    temperature,
                 };
 
                 let stream = model.stream_completion_text(request, &cx);