Use captured step model settings for extension context (#44202)

## Why

Model switches within a turn can leave extension context using stale model metadata. Skill catalogs, context windows, and metric attribution need to match the model captured for each sampling step.

## What changed

- Pass captured model metadata and step-specific telemetry to world-state contributors.
- Use that metadata for skill catalog budgets and usage instructions, preserving configured budget overrides.
- Supply the captured model's usable context window to turn-context contributors, including when rebuilding context.

## Testing

Add regression coverage for model switches during skill discovery, catalog budgets and metric attribution, and extension context windows after `new_context` and retained-step context rebuilds.

GitOrigin-RevId: c51b40b739a380b9767a54ea8c8ee6de45bf01c8

codex-rs/core/src/session/mod.rs

@@ -3964,7 +3964,7 @@ impl Session {
                     session_store: &self.services.session_extension_data,
                     thread_store: &self.services.thread_extension_data,
                     turn_store: turn_context.extension_data.as_ref(),
-                    model_context_window: turn_context.model_context_window(),
+                    model_context_window: step_context.settings.model_info.usable_context_window(),
                 })
                 .await
             {
@@ -4066,7 +4066,7 @@ impl Session {
                     session_store: &self.services.session_extension_data,
                     thread_store: &self.services.thread_extension_data,
                     turn_store: turn_context.extension_data.as_ref(),
-                    model_context_window: turn_context.model_context_window(),
+                    model_context_window: step_context.settings.model_info.usable_context_window(),
                 })
                 .await
             {

codex-rs/core/src/session/tests.rs

@@ -10810,7 +10810,7 @@ async fn record_context_updates_and_set_reference_context_item_persists_split_fi
 
 #[tokio::test]
 async fn build_initial_context_uses_retained_step_after_model_change() {
-    let (session, mut turn_context, _rx_event) =
+    let (mut session, mut turn_context, _rx_event) =
         make_session_and_context_with_auth_and_config_and_rx(
             CodexAuth::from_api_key("Test API Key"),
             Vec::new(),
@@ -10821,6 +10821,17 @@ async fn build_initial_context_uses_retained_step_after_model_change() {
             },
         )
         .await;
+    let mut builder = codex_extension_api::ExtensionRegistryBuilder::new();
+    builder.prompt_contributor(Arc::new(TurnContextExtensionTestContributor));
+    Arc::get_mut(&mut session)
+        .expect("unshared test session")
+        .services
+        .extensions = Arc::new(builder.build());
+    turn_context
+        .extension_data
+        .insert(TurnContextExtensionTestState {
+            expected_model_context_window: Some(64_000),
+        });
     update_turn_settings_for_test(Arc::get_mut(&mut turn_context).unwrap(), |settings| {
         let model_info = Arc::make_mut(&mut settings.model_info);
         model_info.slug = "model-a".to_string();
@@ -10859,6 +10870,7 @@ async fn build_initial_context_uses_retained_step_after_model_change() {
     let mut model_b = step_a.settings.model_info.as_ref().clone();
     model_b.slug = "model-b".to_string();
     model_b.context_window = Some(128_000);
+    model_b.effective_context_window_percent = 50;
     model_b
         .model_messages
         .as_mut()
@@ -10879,6 +10891,7 @@ async fn build_initial_context_uses_retained_step_after_model_change() {
     let initial_b = session
         .build_initial_context_with_world_state(&step_b, &world_b)
         .await;
+    let turn_contributions_b = session.build_turn_context_contribution_items(&step_b).await;
     let (restored_a, restored_world) =
         crate::compact::build_compaction_initial_context(&session, &retained).await;
 
@@ -10894,6 +10907,13 @@ async fn build_initial_context_uses_retained_step_after_model_change() {
     assert!(!a_text.contains("<context_window>"));
     assert!(b_text.contains("B instructions: pragmatic"));
     assert!(!b_text.contains("A instructions:"));
+    assert!(!a_text.contains("turn context extension enabled"));
+    assert!(b_text.contains("turn context extension enabled"));
+    assert!(
+        developer_input_texts(&turn_contributions_b)
+            .join("\n")
+            .contains("turn context extension enabled")
+    );
     assert!(
         b_text.contains("<context_window>"),
         "full-context metadata must use B's window even though the turn started without one"

codex-rs/core/src/session/world_state.rs

@@ -290,13 +290,14 @@ impl Session {
             .map(|root| root.selected_root().clone())
             .collect::<Vec<_>>();
         let extension_metrics = super::extension_metrics::from_session_telemetry(
-            turn_context.session_telemetry.clone(),
+            step_context.session_telemetry.clone(),
         );
         for contributor in self.services.extensions.context_contributors() {
             for section in contributor
                 .contribute_world_state(WorldStateContributionInput {
                     thread_id: self.thread_id(),
                     turn_id: turn_context.sub_id.as_str(),
+                    model_info: &step_context.settings.model_info,
                     environments: &environments,
                     ready_selected_capability_roots: &ready_selected_capability_roots,
                     executor_capability_discovery: step_context