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

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

codex-rs/core/src/models_manager/model_info.rs

@@ -10,8 +10,8 @@ use codex_protocol::openai_models::WebSearchToolType;
 use codex_protocol::openai_models::default_input_modalities;
 
 use crate::config::Config;
-use crate::truncate::approx_bytes_for_tokens;
 use codex_features::Feature;
+use codex_utils_output_truncation::approx_bytes_for_tokens;
 use tracing::warn;
 
 pub const BASE_INSTRUCTIONS: &str = include_str!("../../prompt.md");