Add a turn-scoped service tier override (#40616)

## What changed

- Add `serviceTierForTurn` to `turn/start` so a newly started turn can override the service tier without changing the thread's saved tier.
- Treat `"default"` as standard speed and omitted or `null` values as inheriting the thread tier.
- Keep `serviceTier` authoritative for subsequent turns when both fields are supplied, and ignore the turn-scoped override when steering an active turn.

## Testing

- Extend the app-server turn-start test to verify both the one-turn `"default"` override and inheritance on the following turn.

GitOrigin-RevId: 4c2c4db21d8d31617a7a6e6df65bf54834e72383

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

@@ -788,7 +788,7 @@ impl Session {
         };
 
         let (
-            session_configuration,
+            mut session_configuration,
             mcp_inputs_changed,
             permission_profile_changed,
             previous_config,
@@ -817,6 +817,10 @@ impl Session {
             self.refresh_managed_network_proxy_for_current_permission_profile()
                 .await;
         }
+        // Apply the override only to the turn's copy, after persisting thread settings.
+        if let Some(service_tier) = updates.service_tier_for_turn {
+            session_configuration.service_tier = Some(service_tier);
+        }
         Ok(self
             .new_turn_from_configuration(
                 sub_id,