[codex] prefer inherited spawn agent model (#18701)

This updates the spawn-agent tool contract so subagents are presented as
inheriting the parent model by default. The visible model list is now
framed as optional overrides, the model parameter tells callers to leave
it unset and the delegation guidance no longer nudges models toward
picking a smaller/mini override.

Fixes reports that 5.4 would occasionally pick 5.2 or lower as
sub-agents.

codex-rs/tools/src/agent_tool_tests.rs

@@ -61,6 +61,11 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
         .expect("spawn_agent should use object params");
     assert!(description.contains("Spawns an agent to work on the specified task."));
     assert!(description.contains("The spawned agent will have the same tools as you"));
+    assert!(description.contains(SPAWN_AGENT_INHERITED_MODEL_GUIDANCE));
+    assert!(
+        description
+            .contains("Available model overrides (optional; inherited parent model is preferred):")
+    );
     assert!(description.contains("visible display (`visible-model`)"));
     assert!(!description.contains("hidden display (`hidden-model`)"));
     assert!(properties.contains_key("task_name"));
@@ -72,6 +77,12 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
         properties.get("agent_type"),
         Some(&JsonSchema::string(Some("role help".to_string())))
     );
+    assert_eq!(
+        properties
+            .get("model")
+            .and_then(|schema| schema.description.as_deref()),
+        Some(SPAWN_AGENT_MODEL_OVERRIDE_DESCRIPTION)
+    );
     assert_eq!(
         parameters.required.as_ref(),
         Some(&vec!["task_name".to_string(), "message".to_string()])
@@ -106,6 +117,12 @@ fn spawn_agent_tool_v1_keeps_legacy_fork_context_field() {
 
     assert!(properties.contains_key("fork_context"));
     assert!(!properties.contains_key("fork_turns"));
+    assert_eq!(
+        properties
+            .get("model")
+            .and_then(|schema| schema.description.as_deref()),
+        Some(SPAWN_AGENT_MODEL_OVERRIDE_DESCRIPTION)
+    );
 }
 
 #[test]