Keep active-turn model settings stable across updates (#38785)

## Why

Thread settings can change while a turn is still active, including between sampling requests. Those updates should apply to the next turn instead of changing the model configuration partway through the current turn.

## What changed

- Snapshot the model, reasoning settings, service tier, approval settings, and model-attributed telemetry in `StepContext`.
- Build prompts, model requests, world state, tracing, and startup prewarms from that step-scoped snapshot.
- Share immutable model metadata between turn and step contexts.

## Testing

Added an integration test that pauses an active turn, updates its thread settings, and verifies that all requests in the active turn retain the original settings while the next turn uses the updates.

GitOrigin-RevId: c51352218e463dcc9b7338566d16742292c49fbf

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

@@ -138,12 +138,18 @@ pub struct TurnContext {
     pub(crate) trace_id: Option<String>,
     pub(crate) realtime_active: bool,
     pub(crate) code_mode_available: bool,
+    /// Turn-scoped configuration. Read step-specific settings such as service tier and
+    /// approvals reviewer from the corresponding `StepContext` fields instead.
     pub config: Arc<Config>,
     pub(crate) auth_manager: Option<Arc<AuthManager>>,
-    pub(crate) model_info: ModelInfo,
+    /// Legacy turn model; step-scoped execution should use `StepContext::model_info`.
+    pub(crate) model_info: Arc<ModelInfo>,
+    /// Turn-wide telemetry; model-attributed step work should use `StepContext::session_telemetry`.
     pub(crate) session_telemetry: SessionTelemetry,
     pub(crate) provider: SharedModelProvider,
+    /// Legacy turn effort; step-scoped execution should use `StepContext::reasoning_effort`.
     pub(crate) reasoning_effort: Option<ReasoningEffortConfig>,
+    /// Legacy turn summary; step-scoped execution should use `StepContext::reasoning_summary`.
     pub(crate) reasoning_summary: ReasoningSummaryConfig,
     pub(crate) session_source: SessionSource,
     pub(crate) history_mode: ThreadHistoryMode,
@@ -223,6 +229,8 @@ impl TurnContext {
             .await
     }
 
+    /// Approval policy admitted with the turn; step-scoped actions should use
+    /// `StepContext::approval_policy` instead.
     pub(crate) fn approval_policy(&self) -> AskForApproval {
         self.config.permissions.approval_policy.value()
     }
@@ -352,6 +360,7 @@ impl TurnContext {
                 config.http_client_factory(),
             )
             .await;
+        let model_info = Arc::new(model_info);
 
         Self {
             sub_id: self.sub_id.clone(),
@@ -360,7 +369,7 @@ impl TurnContext {
             code_mode_available: self.code_mode_available,
             config: Arc::new(config),
             auth_manager: self.auth_manager.clone(),
-            model_info: model_info.clone(),
+            model_info: Arc::clone(&model_info),
             session_telemetry: self
                 .session_telemetry
                 .clone()
@@ -643,7 +652,7 @@ impl Session {
             code_mode_available: true,
             config: per_turn_config,
             auth_manager,
-            model_info,
+            model_info: Arc::new(model_info),
             session_telemetry: session_telemetry_for_context,
             provider,
             reasoning_effort,