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_input.rs

@@ -98,12 +98,14 @@ impl PreparedTurnInputSettings {
     ) -> CodexResult<Arc<TurnContext>> {
         let TurnStartOptions {
             final_output_json_schema,
+            service_tier,
             parent_turn_id,
             root_turn_id,
         } = self.start_options;
         let emit_thread_settings_applied = self.thread_settings_update.is_some();
         let mut updates = self.thread_settings_update.unwrap_or_default();
         updates.final_output_json_schema = Some(final_output_json_schema);
+        updates.service_tier_for_turn = service_tier;
 
         // new_turn_with_sub_id already emits an error event when settings are invalid.
         let turn_context = session