Add live turn settings updates (#40653)

## What changed

- Add a feature-gated `TurnSettings` operation that can update the model,
  reasoning effort, reasoning summary, and service tier for subsequent steps of
  a named running turn without changing future thread settings.
- Report whether an update was applied, rejected, or lost its live target.
- Revalidate managed constraints and preserve the turn's admitted approval and
  Guardian safety properties before publishing an updated settings snapshot.

## Testing

- Cover sparse updates, step capture and ordering, target replacement, managed
  policy changes, model metadata safety, and separation from future settings.

GitOrigin-RevId: ecaaaa95b4fd4ec9d62265bef551582dddb36d78

codex-rs/core/config.schema.json

@@ -907,6 +907,9 @@
             "steer": {
               "type": "boolean"
             },
+            "step_model_switching": {
+              "type": "boolean"
+            },
             "telepathy": {
               "type": "boolean"
             },
@@ -5916,6 +5919,9 @@
         "steer": {
           "type": "boolean"
         },
+        "step_model_switching": {
+          "type": "boolean"
+        },
         "telepathy": {
           "type": "boolean"
         },