Snapshot resolved settings for each model step (#40651)

## Why

Thread settings can change while a turn is running, and delegated review or
compaction steps can select a different model. Each request needs a consistent
set of settings resolved against the model that will execute it.

## What changed

- Capture immutable resolved settings for each model step, including model
  metadata, reasoning options, service tier, approvals, and personality.
- Keep in-flight steps on their captured settings while applying thread updates
  to subsequent work.
- Resolve inherited reasoning summaries and filter service tiers against the
  selected model and feature configuration for review and compaction steps.

## Testing

Added coverage for settings updates during paused turns, model changes,
previous-model compaction, review-model defaults, and service-tier filtering.

GitOrigin-RevId: fca4b3b305379f1bf26a5cac75ee0dd1747ac1f6

codex-rs/core/src/stream_events_utils.rs

@@ -100,7 +100,7 @@ pub(crate) async fn record_completed_response_item_with_finalized_facts(
         || {
             completed_item_defers_mailbox_delivery_to_next_turn(
                 item,
-                turn_context.mode == ModeKind::Plan,
+                turn_context.mode() == ModeKind::Plan,
             )
         },
         |facts| facts.defers_mailbox_delivery_to_next_turn,
@@ -292,7 +292,7 @@ pub(crate) async fn handle_output_item_done(
     previously_active_item: Option<TurnItem>,
 ) -> Result<OutputItemResult> {
     let mut output = OutputItemResult::default();
-    let plan_mode = ctx.turn_context.mode == ModeKind::Plan;
+    let plan_mode = ctx.turn_context.mode() == ModeKind::Plan;
 
     match ToolRouter::build_tool_call(item.clone()) {
         // The model emitted a tool call; log it, persist the item immediately, and queue the tool execution.