assistant2: Allow creating agent profiles via settings (#27216)

This PR adds support for creating new agent profiles via the settings:

```json
{
  "assistant": {
    "profiles": {
      "lua": {
        "name": "Lua",
        "tools": {
          "lua-interpreter": true
        }
      },
      "lua-thinking": {
        "name": "Lua + Thinking",
        "tools": {
          "lua-interpreter": true,
          "thinking": true
        }
      }
    }
  }
}
```

Release Notes:

- N/A

crates/assistant_settings/Cargo.toml

@@ -14,6 +14,7 @@ path = "src/assistant_settings.rs"
 [dependencies]
 anthropic = { workspace = true, features = ["schemars"] }
 anyhow.workspace = true
+collections.workspace = true
 feature_flags.workspace = true
 gpui.workspace = true
 language_model.workspace = true