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/truncate.rs

@@ -235,7 +235,7 @@ fn truncate_with_byte_estimate(s: &str, max_bytes: usize, source: TruncationSour
     out
 }
 
-fn truncate_formatted_exec_output(
+pub(crate) fn truncate_formatted_exec_output(
     content: &str,
     total_lines: usize,
     limit_bytes: usize,