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/exec/src/lib.rs

@@ -986,6 +986,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
                         permissions: None,
                         model: None,
                         service_tier: None,
+                        service_tier_for_turn: None,
                         effort: default_effort,
                         summary: None,
                         personality: None,