agent: Add ability to set a default_model per profile (#39220)

Split off from https://github.com/zed-industries/zed/pull/39175

Requires https://github.com/zed-industries/zed/pull/39219 to be merged
first

Adds support for `default_model` for profiles: 

```
      "my-profile": {
        "name": "Coding Agent",
        "tools": {},
        "enable_all_context_servers": false,
        "context_servers": {},
        "default_model": {
          "provider": "copilot_chat",
          "model": "grok-code-fast-1"
        }
      }
```

Which will then switch to the default model whenever the profile is
activated

![2025-09-30 17 09
06](https://github.com/user-attachments/assets/43f07b7b-85d9-4aff-82ce-25d6f5050d50)


Release Notes:

- Added `default_model` configuration to agent profile

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/settings/src/settings_content/agent.rs

@@ -176,6 +176,8 @@ pub struct AgentProfileContent {
     pub enable_all_context_servers: Option<bool>,
     #[serde(default)]
     pub context_servers: IndexMap<Arc<str>, ContextServerPresetContent>,
+    /// The default language model selected when using this profile.
+    pub default_model: Option<LanguageModelSelection>,
 }
 
 #[skip_serializing_none]