Ensure exec command end always emitted (#1908)

## Summary
- defer ExecCommandEnd emission until after sandbox resolution
- make sandbox error handler return final exec output and response
- align sandbox error stderr with response content and rename to
`final_output`
- replace unstable `let` chains in client command header logic

## Testing
- `just fmt`
- `just fix`
- `cargo test --all-features` *(fails: NotPresent in
core/tests/client.rs)*

------
https://chatgpt.com/codex/tasks/task_i_6893e63b0c408321a8e1ff2a052c4c51

codex-rs/core/src/error.rs

@@ -132,3 +132,10 @@ impl CodexErr {
         (self as &dyn std::any::Any).downcast_ref::<T>()
     }
 }
+
+pub fn get_error_message_ui(e: &CodexErr) -> String {
+    match e {
+        CodexErr::Sandbox(SandboxErr::Denied(_, _, stderr)) => stderr.to_string(),
+        _ => e.to_string(),
+    }
+}