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/src/tools/handlers/multi_agents_spec.rs

@@ -709,7 +709,7 @@ fn spawn_agent_tool_description(
     format!(
         r#"
         {tool_description}
-This spawn_agent tool provides you access to sub-agents that inherit your current model by default. Do not set the `model` field unless the user explicitly asks for a different model or there is a clear task-specific reason. You should follow the rules and guidelines below to use this tool.
+This spawn_agent tool provides you access to sub-agents that inherit your current model by default. Do not set the `model` field unless the user explicitly asks for a different model. You should follow the rules and guidelines below to use this tool.
 
 Do not spawn sub-agents unless the user or applicable AGENTS.md/skill instructions explicitly ask for sub-agents, delegation, or parallel agent work.
 Requests for depth, thoroughness, research, investigation, or detailed codebase analysis do not count as permission to spawn.

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:?}"
     );