Replace unreachable!() with proper error in retry loop

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/oneshot.rs

@@ -314,7 +314,7 @@ impl AutoAgent {
                 }
             }
         }
-        unreachable!()
+        Err(format!("{}: all retry attempts exhausted", name))
     }
 
     async fn dispatch_tools(&mut self, backend: &mut Backend, msg: &Message) {