chore: implement Mul for TruncationPolicy (#10272)

Codex thought this was a good idea while working on
https://github.com/openai/codex/pull/10192.

codex-rs/core/src/context_manager/history.rs

@@ -266,7 +266,7 @@ impl ContextManager {
     }
 
     fn process_item(&self, item: &ResponseItem, policy: TruncationPolicy) -> ResponseItem {
-        let policy_with_serialization_budget = policy.mul(1.2);
+        let policy_with_serialization_budget = policy * 1.2;
         match item {
             ResponseItem::FunctionCallOutput { call_id, output } => {
                 let truncated =

codex-rs/core/src/truncate.rs

@@ -34,18 +34,6 @@ impl From<TruncationPolicyConfig> for TruncationPolicy {
 }
 
 impl TruncationPolicy {
-    /// Scale the underlying budget by `multiplier`, rounding up to avoid under-budgeting.
-    pub fn mul(self, multiplier: f64) -> Self {
-        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)
-            }
-        }
-    }
-
     /// Returns a token budget derived from this policy.
     ///
     /// - For `Tokens`, this is the explicit token limit.
@@ -73,6 +61,21 @@ impl TruncationPolicy {
     }
 }
 
+impl std::ops::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)
+            }
+        }
+    }
+}
+
 pub(crate) fn formatted_truncate_text(content: &str, policy: TruncationPolicy) -> String {
     if content.len() <= policy.byte_budget() {
         return content.to_string();