Forward truncation policies to the history notes backend (#41062)

## What changed

- Serialize each history and notes request's output truncation policy into the
  `x-openai-tool-output-truncation-policy` header.
- Forward the invoking tool call's policy for tool requests and use the thread
  hint byte limit for context-contributor requests.

GitOrigin-RevId: 9d4e61480397b325efdb033861893e88c708de3b

codex-rs/ext/history-notes/src/backend.rs

@@ -5,6 +5,7 @@ use codex_client::HttpTransport;
 use codex_client::RequestBody;
 use codex_login::default_client::create_client;
 use codex_model_provider::SharedModelProvider;
+use codex_utils_output_truncation::TruncationPolicy;
 use http::HeaderValue;
 use http::Method;
 use serde_json::Value;
@@ -12,6 +13,7 @@ use serde_json::json;
 
 const HISTORY_NOTES_BACKEND_TIMEOUT: Duration = Duration::from_secs(35);
 const ENCRYPTED_TOOL_ARGUMENTS_HEADER: &str = "x-openai-encrypted-tool-arguments";
+const TOOL_OUTPUT_TRUNCATION_POLICY_HEADER: &str = "x-openai-tool-output-truncation-policy";
 
 #[derive(Clone)]
 pub(crate) struct HistoryNotesBackend {
@@ -29,6 +31,7 @@ impl HistoryNotesBackend {
         session_id: &str,
         current_agent_name: &str,
         mut arguments: Value,
+        truncation_policy: TruncationPolicy,
     ) -> Result<Value, String> {
         let Some(arguments_object) = arguments.as_object_mut() else {
             return Err("History tool arguments must be a JSON object".to_string());
@@ -52,6 +55,14 @@ impl HistoryNotesBackend {
             .map_err(|error| format!("History backend auth could not be resolved: {error}"))?;
 
         let mut request = provider.build_request(Method::POST, path);
+        let encoded_truncation_policy = serde_json::to_string(&truncation_policy)
+            .map_err(|error| format!("Could not encode tool output truncation policy: {error}"))?;
+        request.headers.insert(
+            TOOL_OUTPUT_TRUNCATION_POLICY_HEADER,
+            HeaderValue::from_str(&encoded_truncation_policy).map_err(|error| {
+                format!("Invalid tool output truncation policy header: {error}")
+            })?,
+        );
         if matches!(
             path,
             "alpha/history/v2/search_contents"