shell_command returns freeform output (#6860)

Instead of returning structured out and then re-formatting it into
freeform, return the freeform output from shell_command tool.

Keep `shell` as the default tool for GPT-5.

codex-rs/core/src/client_common.rs

@@ -136,7 +136,7 @@ fn reserialize_shell_outputs(items: &mut [ResponseItem]) {
 }
 
 fn is_shell_tool_name(name: &str) -> bool {
-    matches!(name, "shell" | "container.exec" | "shell_command")
+    matches!(name, "shell" | "container.exec")
 }
 
 #[derive(Deserialize)]