Enable retained-image budgeting by default (#40994)

## What changed

- Promote `compaction_image_budget` to stable and enable it by default.
- Charge retained images against the remote compaction context budget unless the feature is explicitly disabled, trimming older images as needed.

## Testing

- Update remote compaction coverage for the default, explicitly enabled, and explicitly disabled behavior.

GitOrigin-RevId: 0f715347cefc33b814bdcfd9b430ef453d3bd6df

codex-rs/core/tests/suite/compact_remote.rs

@@ -1332,7 +1332,7 @@ async fn remote_manual_compact_chatgpt_auth_reuses_service_tier_and_prompt_cache
     Ok(())
 }
 
-#[test_case(None; "default_preserves_images")]
+#[test_case(None; "default_trims_images")]
 #[test_case(Some(false); "disabled_preserves_images")]
 #[test_case(Some(true); "enabled_trims_images")]
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
@@ -1437,7 +1437,7 @@ async fn remote_compact_v2_charges_retained_images_to_token_budget(
             follow_up.inputs_of_type("compaction")[0]["encrypted_content"],
             "IMAGE_BUDGET_SUMMARY"
         );
-        let dropped = if image_budget_enabled == Some(true) {
+        let dropped = if image_budget_enabled.unwrap_or(true) {
             cycle
         } else {
             0

codex-rs/features/src/lib.rs

@@ -1616,8 +1616,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::CompactionImageBudget,
         key: "compaction_image_budget",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Stable,
-        default_enabled: false,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::RetainClientDeveloperMessages,