Require explicit requests for spawn model overrides (#41165)

## What changed

- Tell the `spawn_agent` tool to set `model` only when the user explicitly
  requests a different model.
- Update the tool-description test to enforce the stricter guidance.

GitOrigin-RevId: ed46bcb544876e408896e0d8c54292192369951c

codex-rs/core/tests/suite/spawn_agent_description.rs

@@ -235,7 +235,7 @@ async fn spawn_agent_description_lists_visible_models_and_reasoning_efforts() ->
     );
     assert!(
         description.contains(
-            "Do not set the `model` field unless the user explicitly asks for a different model or there is a clear task-specific reason."
+            "Do not set the `model` field unless the user explicitly asks for a different model."
         ),
         "expected model override usage guidance in spawn_agent description: {description:?}"
     );