Cycle message roles on click

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/ai/src/ai.rs

@@ -34,6 +34,16 @@ enum Role {
     System,
 }
 
+impl Role {
+    pub fn cycle(&mut self) {
+        *self = match self {
+            Role::User => Role::Assistant,
+            Role::Assistant => Role::System,
+            Role::System => Role::User,
+        }
+    }
+}
+
 impl Display for Role {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> std::fmt::Result {
         match self {