Move string truncation helpers into codex-utils-string (#15572)

- move the shared byte-based middle truncation logic from `core` into
`codex-utils-string`
- keep token-specific truncation in `codex-core` so rollout can reuse
the shared helper in the next stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/error.rs

@@ -2,8 +2,6 @@ use crate::exec::ExecToolCallOutput;
 use crate::network_policy_decision::NetworkPolicyDecisionPayload;
 use crate::token_data::KnownPlan;
 use crate::token_data::PlanType;
-use crate::truncate::TruncationPolicy;
-use crate::truncate::truncate_text;
 use chrono::DateTime;
 use chrono::Datelike;
 use chrono::Local;
@@ -15,6 +13,8 @@ use codex_protocol::ThreadId;
 use codex_protocol::protocol::CodexErrorInfo;
 use codex_protocol::protocol::ErrorEvent;
 use codex_protocol::protocol::RateLimitSnapshot;
+use codex_utils_output_truncation::TruncationPolicy;
+use codex_utils_output_truncation::truncate_text;
 use reqwest::StatusCode;
 use serde_json;
 use std::io;