Expose model overrides for multi-agent v2 spawns (#32749)

## What changed

- Add the `features.multi_agent_v2.expose_spawn_agent_model_overrides` setting, enabled by default, to expose `model` and `reasoning_effort` on the v2 `spawn_agent` tool.
- Keep these controls available when other spawn metadata is hidden, while allowing them to be disabled independently.
- Add root-agent and subagent guidance that overrides require a partial or context-free fork and should only be used when explicitly authorized.

## Testing

- Cover configuration parsing and defaults, usage-hint preservation, and tool-schema behavior with override exposure enabled and disabled.

GitOrigin-RevId: 92370498108c96fbd51f32965624ce531e991d9a

codex-rs/features/src/tests.rs

@@ -651,6 +651,7 @@ subagent_usage_hint_text = "Subagent guidance."
 multi_agent_mode_hint_text = "Custom mode guidance."
 tool_namespace = "agents"
 hide_spawn_agent_metadata = true
+expose_spawn_agent_model_overrides = true
 non_code_mode_only = true
 "#,
     )
@@ -675,6 +676,7 @@ non_code_mode_only = true
             multi_agent_mode_hint_text: Some("Custom mode guidance.".to_string()),
             tool_namespace: Some("agents".to_string()),
             hide_spawn_agent_metadata: Some(true),
+            expose_spawn_agent_model_overrides: Some(true),
             non_code_mode_only: Some(true),
         }))
     );