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/collab/src/tests/test_server.rs

@@ -307,6 +307,7 @@ impl TestServer {
             );
             language_model::LanguageModelRegistry::test(cx);
             assistant_context_editor::init(client.clone(), cx);
+            assistant_settings::init(cx);
         });
 
         client