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/app-server-protocol/src/protocol/v2/tests.rs

@@ -4725,6 +4725,7 @@ fn turn_start_params_preserve_explicit_null_service_tier() {
         permissions: None,
         model: None,
         service_tier: None,
+        service_tier_for_turn: None,
         effort: None,
         summary: None,
         output_schema: None,

codex-rs/app-server-protocol/src/protocol/v2/turn.rs

@@ -131,6 +131,11 @@ pub struct TurnStartParams {
     )]
     #[ts(optional = nullable)]
     pub service_tier: Option<Option<String>>,
+    /// Override the service tier only when this request starts a new turn.
+    /// Use "default" for standard speed. Omitted or null inherits the thread's tier.
+    /// Does not change the thread's tier or a turn being steered.
+    #[ts(optional = nullable)]
+    pub service_tier_for_turn: Option<String>,
     /// Override the reasoning effort for this turn and subsequent turns.
     #[ts(optional = nullable)]
     pub effort: Option<ReasoningEffort>,