Fix unified exec test output assertion (#14184)

## Summary
- update the unified exec test to use truncated_output() instead of the
removed output field
- fix the compile failure on latest main after ExecCommandToolOutput
changed shape

codex-rs/core/src/unified_exec/mod.rs

@@ -444,7 +444,7 @@ mod tests {
             "pause should block the unified exec yield timeout"
         );
         assert!(
-            response.output.contains("unified-exec-done"),
+            response.truncated_output().contains("unified-exec-done"),
             "exec_command should wait for output after the pause lifts"
         );
         assert!(