agent: Add newtype for profile IDs (#27939)

This PR adds an `AgentProfileId` newtype for profile IDs that we can use
instead of `Arc<str>` everywhere.

Release Notes:

- N/A

crates/assistant_settings/src/agent_profile.rs

@@ -2,6 +2,29 @@ use std::sync::Arc;
 
 use gpui::SharedString;
 use indexmap::IndexMap;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize, JsonSchema)]
+pub struct AgentProfileId(pub Arc<str>);
+
+impl AgentProfileId {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl std::fmt::Display for AgentProfileId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl Default for AgentProfileId {
+    fn default() -> Self {
+        Self("write".into())
+    }
+}
 
 /// A profile for the Zed Agent that controls its behavior.
 #[derive(Debug, Clone)]