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/schema/json/codex_app_server_protocol.schemas.json

@@ -23939,6 +23939,13 @@
               "null"
             ]
           },
+          "serviceTierForTurn": {
+            "description": "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.",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "summary": {
             "anyOf": [
               {