Expose spawn agent types only when roles are configured (#33572)

## Why

The `agent_type` argument is only useful when agent roles are configured. Avoid
advertising an inapplicable override in the model-visible `spawn_agent` schema.

## What changed

- Include `agent_type` in the V1 and V2 spawn tools only when configured roles
  are available.
- Clarify that `agent_type` is an explicit override and document its interaction
  with context forking.
- Remove the unused `agents.support_agent_type` configuration field.

## Testing

- Add coverage for hiding `agent_type` without roles and exposing it when a role
  is configured.

GitOrigin-RevId: 32e0f123b15f317e9cdb71774029375bc073d9ad

codex-rs/core/src/tools/spec_plan.rs

@@ -798,16 +798,15 @@ fn add_collaboration_tools(context: &CoreToolPlanContext<'_>, planned_tools: &mu
                 .flatten();
             let agent_type_description =
                 agent_type_description(turn_context, context.default_agent_type_description);
-            // TODO(jif): Wire `agents.support_agent_type` into the V2 spawn tool surface.
+            let hide_spawn_agent_metadata =
+                turn_context.config.multi_agent_v2.hide_spawn_agent_metadata;
             planned_tools.add_arc(override_tool_exposure(
                 multi_agent_v2_handler(
                     SpawnAgentHandlerV2::new(SpawnAgentToolOptions {
                         available_models: turn_context.available_models.clone(),
                         agent_type_description,
-                        hide_agent_type_model_reasoning: turn_context
-                            .config
-                            .multi_agent_v2
-                            .hide_spawn_agent_metadata,
+                        expose_agent_type: !turn_context.config.agent_roles.is_empty(),
+                        hide_agent_type_model_reasoning: hide_spawn_agent_metadata,
                         expose_spawn_agent_model_overrides: turn_context
                             .config
                             .multi_agent_v2
@@ -854,6 +853,7 @@ fn add_collaboration_tools(context: &CoreToolPlanContext<'_>, planned_tools: &mu
                 SpawnAgentHandler::new(SpawnAgentToolOptions {
                     available_models: turn_context.available_models.clone(),
                     agent_type_description,
+                    expose_agent_type: !turn_context.config.agent_roles.is_empty(),
                     hide_agent_type_model_reasoning: false,
                     expose_spawn_agent_model_overrides: true,
                     multi_agent_version: turn_context.multi_agent_version,