Refactor truncation helpers into its own file (#6683)

That's to centralize the truncation in one place. Next step would be to
make only two methods public: one with bytes/lines and one with tokens.

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

@@ -124,37 +124,6 @@ pub(crate) fn generate_chunk_id() -> String {
         .collect()
 }
 
-pub(crate) fn truncate_output_to_tokens(
-    output: &str,
-    max_tokens: usize,
-) -> (String, Option<usize>) {
-    if max_tokens == 0 {
-        let total_tokens = output.chars().count();
-        let message = format!("…{total_tokens} tokens truncated…");
-        return (message, Some(total_tokens));
-    }
-
-    let tokens: Vec<char> = output.chars().collect();
-    let total_tokens = tokens.len();
-    if total_tokens <= max_tokens {
-        return (output.to_string(), None);
-    }
-
-    let half = max_tokens / 2;
-    if half == 0 {
-        let truncated = total_tokens.saturating_sub(max_tokens);
-        let message = format!("…{truncated} tokens truncated…");
-        return (message, Some(total_tokens));
-    }
-
-    let truncated = total_tokens.saturating_sub(half * 2);
-    let mut truncated_output = String::new();
-    truncated_output.extend(&tokens[..half]);
-    truncated_output.push_str(&format!("…{truncated} tokens truncated…"));
-    truncated_output.extend(&tokens[total_tokens - half..]);
-    (truncated_output, Some(total_tokens))
-}
-
 #[cfg(test)]
 #[cfg(unix)]
 mod tests {

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

@@ -36,7 +36,7 @@ use super::generate_chunk_id;
 use super::resolve_max_tokens;
 use super::session::OutputBuffer;
 use super::session::UnifiedExecSession;
-use super::truncate_output_to_tokens;
+use crate::truncate::truncate_output_to_tokens;
 
 impl UnifiedExecSessionManager {
     pub(crate) async fn exec_command(