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>

Cargo.lock

@@ -595,6 +595,7 @@ version = "0.1.0"
 dependencies = [
  "anthropic",
  "anyhow",
+ "collections",
  "deepseek",
  "feature_flags",
  "fs",
@@ -3006,6 +3007,7 @@ dependencies = [
  "anyhow",
  "assistant",
  "assistant_context_editor",
+ "assistant_settings",
  "assistant_slash_command",
  "assistant_tool",
  "async-stripe",