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/unified_exec/mod_tests.rs

@@ -193,7 +193,7 @@ async fn exec_command_with_tty(
         chunk_id: generate_chunk_id(),
         wall_time,
         raw_output: collected,
-        truncation_policy: turn.model_info.truncation_policy.into(),
+        truncation_policy: turn.model_info().truncation_policy.into(),
         max_output_tokens: None,
         process_id: response_process_id,
         exit_code,

codex-rs/core/src/unified_exec/process_manager.rs

@@ -741,7 +741,7 @@ impl UnifiedExecProcessManager {
             truncation_policy: context
                 .step_context
                 .turn
-                .model_info
+                .model_info()
                 .truncation_policy
                 .into(),
             max_output_tokens: request.max_output_tokens,