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

Release Notes:
- Added `default_model` configuration to agent profile
---------
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>