Retain client-marked tool outputs across compaction (#52937)

## Why

Client-supplied tool outputs can carry task instructions. Even when marked with `retain: true`, these instructions could be trimmed before compaction and disappear from the compacted history.

## What changed

Add the disabled-by-default `retain_client_tool_outputs` feature to honor retention for explicitly marked, named `function_call_output` items without a `call_id`.

- Preserve eligible outputs in remote compaction v2 and when starting a new context window, keeping their original role.
- Stop emergency tool-output trimming at a retained output.
- Share the existing 64,000-token retention budget, keeping outputs whole and dropping an oversized output and older items rather than partially truncating it.
- Place initial context before retained standalone outputs and account for their images in retained-image counts.

## Testing

Add a scenario covering enabled and disabled retention, emergency trimming, oversized-output handling, and preservation of the latest instruction after compaction and cold resume. Verify that retention annotations stay out of model requests.

GitOrigin-RevId: 449179670e4d2be2623928700738758e43b5dc1a

codex-rs/features/src/lib.rs

@@ -423,6 +423,8 @@ pub enum Feature {
     CompactionImageBudget,
     /// Retain client-authored developer messages across compacted context windows.
     RetainClientDeveloperMessages,
+    /// Honor client-requested tool-output retention across context windows.
+    RetainClientToolOutputs,
     /// Use Agent Identity for ChatGPT-authenticated sessions.
     UseAgentIdentity,
     /// Enable workspace dependency support.
@@ -2052,6 +2054,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::RetainClientToolOutputs,
+        key: "retain_client_tool_outputs",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::UseAgentIdentity,
         key: "use_agent_identity",