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/agent/src/active_thread.rs

@@ -1417,7 +1417,10 @@ impl ActiveThread {
                         messages: vec![request_message],
                         tools: vec![],
                         stop: vec![],
-                        temperature: None,
+                        temperature: AssistantSettings::temperature_for_model(
+                            &configured_model.model,
+                            cx,
+                        ),
                     };
 
                     Some(configured_model.model.count_tokens(request, cx))