settings ui: Create settings key trait (#37489)

This PR separates out the associated constant `KEY` from the `Settings`
trait into a new trait `SettingsKey`. This allows for the key trait to
be derived using attributes to specify the path so that the new
`SettingsUi` derive macro can use the same attributes to determine top
level settings paths thereby removing the need to duplicate the path in
both `Settings::KEY` and `#[settings_ui(path = "...")]`

Co-authored-by: Ben Kunkle <ben@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/agent_settings/src/agent_settings.rs

@@ -8,7 +8,7 @@ use gpui::{App, Pixels, SharedString};
 use language_model::LanguageModel;
 use schemars::{JsonSchema, json_schema};
 use serde::{Deserialize, Serialize};
-use settings::{Settings, SettingsSources, SettingsUi};
+use settings::{Settings, SettingsKey, SettingsSources, SettingsUi};
 use std::borrow::Cow;
 
 pub use crate::agent_profile::*;
@@ -223,7 +223,8 @@ impl AgentSettingsContent {
     }
 }
 
-#[derive(Clone, Serialize, Deserialize, JsonSchema, Debug, Default, SettingsUi)]
+#[derive(Clone, Serialize, Deserialize, JsonSchema, Debug, Default, SettingsUi, SettingsKey)]
+#[settings_key(key = "agent", fallback_key = "assistant")]
 pub struct AgentSettingsContent {
     /// Whether the Agent is enabled.
     ///
@@ -399,10 +400,6 @@ pub struct ContextServerPresetContent {
 }
 
 impl Settings for AgentSettings {
-    const KEY: Option<&'static str> = Some("agent");
-
-    const FALLBACK_KEY: Option<&'static str> = Some("assistant");
-
     const PRESERVED_KEYS: Option<&'static [&'static str]> = Some(&["version"]);
 
     type FileContent = AgentSettingsContent;