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/protocol/src/protocol.rs

@@ -7,6 +7,7 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 use std::ffi::OsStr;
 use std::fmt;
+use std::ops::Mul;
 use std::path::Path;
 use std::path::PathBuf;
 use std::str::FromStr;
@@ -2629,6 +2630,51 @@ pub enum TruncationPolicy {
     Tokens(usize),
 }
 
+impl From<crate::openai_models::TruncationPolicyConfig> for TruncationPolicy {
+    fn from(config: crate::openai_models::TruncationPolicyConfig) -> Self {
+        match config.mode {
+            crate::openai_models::TruncationMode::Bytes => Self::Bytes(config.limit as usize),
+            crate::openai_models::TruncationMode::Tokens => Self::Tokens(config.limit as usize),
+        }
+    }
+}
+
+impl TruncationPolicy {
+    pub fn token_budget(&self) -> usize {
+        match self {
+            TruncationPolicy::Bytes(bytes) => {
+                usize::try_from(codex_utils_string::approx_tokens_from_byte_count(*bytes))
+                    .unwrap_or(usize::MAX)
+            }
+            TruncationPolicy::Tokens(tokens) => *tokens,
+        }
+    }
+
+    pub fn byte_budget(&self) -> usize {
+        match self {
+            TruncationPolicy::Bytes(bytes) => *bytes,
+            TruncationPolicy::Tokens(tokens) => {
+                codex_utils_string::approx_bytes_for_tokens(*tokens)
+            }
+        }
+    }
+}
+
+impl Mul<f64> for TruncationPolicy {
+    type Output = Self;
+
+    fn mul(self, multiplier: f64) -> Self::Output {
+        match self {
+            TruncationPolicy::Bytes(bytes) => {
+                TruncationPolicy::Bytes((bytes as f64 * multiplier).ceil() as usize)
+            }
+            TruncationPolicy::Tokens(tokens) => {
+                TruncationPolicy::Tokens((tokens as f64 * multiplier).ceil() as usize)
+            }
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Clone, JsonSchema)]
 pub struct RolloutLine {
     pub timestamp: String,